synthesis and characterisation of covalent organic ... · organic frameworks that were...

SYNTHESIS AND

CHARACTERISATION OF

COVALENT ORGANIC

FRAMEWORKS AS THIN FILMS

Nikka Maria Joezar Turangan

B. App. Sci.

Submitted in fulfilment of the requirements for the degree of

Master of Applied Science

School of Chemistry, Physics and Mechanical Engineering

Science and Engineering Faculty

Queensland University of Technology

2019

SYNTHESIS AND CHARACTERISATION OF COVALENT ORGANIC FRAMEWORKS AS THIN FILMS i

Keywords

Covalent organic frameworks, COF, porous material, membrane, thin films, self-

supporting, freestanding, condensation reaction, reversibility, dynamic covalent

chemistry

ii SYNTHESIS AND CHARACTERISATION OF COVALENT ORGANIC FRAMEWORKS AS THIN FILMS

Abstract

Covalent organic frameworks (COFs) were polymer assemblies formed from

light elements with high crystallinity, high porosity and configurable skeletal structure.

They have potential applications in filtration, gas storage, and electronic devices

among others. However, the realization of these applications requires a high degree of

morphological control in their preparation. The construction of substrate-confined

COF materials that were simultaneously highly crystalline, well-oriented and

functional remains a challenge due to the relatively small amount of work that has been

done to study the nucleation and growth of covalent organic frameworks, particularly

on a substrate. An understanding of COF growth in films is crucial to further progress

the development of more concrete techniques that will ultimately lead to covalent

organic frameworks that were crystallographically and structurally well-defined in

film form. A similar challenge exists around freestanding COF membranes,

which could be used for flow-through application or integrated to other systems. In

this thesis, a range of synthesis techniques to fabricate substrate-confined films were

explored, and procedures to create freestanding films were constructed. We began the

project with preliminary studies of known thin film synthesis methods that we then

adapted for precursor film fabrication. Through this we improved on the quality of

crucial parameters and further categorized our refined options based on different

strategies: thermal annealing and room temperature solvent-vapour annealing. These

intensive experiments led to the development of a procedure that requires no thermal

input for the creation of freestanding films. We found solvent choice and precursor

concentration to be the key parameters for film formation, whether substrate-confined

or self-supporting.

SYNTHESIS AND CHARACTERISATION OF COVALENT ORGANIC FRAMEWORKS AS THIN FILMS iii

Table of Contents

Keywords .................................................................................................................................. i

Abstract .................................................................................................................................... ii

Table of Contents .................................................................................................................... iii

List of Figures ........................................................................................................................... v

List of Tables ............................................................................................................................ x

List of Abbreviations .............................................................................................................. xi

Statement of Original Authorship .......................................................................................... xii

Dedication ............................................................................................................................. xiii

Acknowledgements ................................................................................................................. xv

Chapter 1: Introduction ...................................................................................... 1

1.1 Background ..................................................................................................................... 1

1.2 Context............................................................................................................................ 2

1.3 Purpose ........................................................................................................................... 2

1.4 Significance, Scope and Definitions ............................................................................... 2

1.5 Thesis Outline ................................................................................................................. 3

Chapter 2: Literature Review ............................................................................. 5

2.1 What Makes a Porous Material? ..................................................................................... 5

2.2 Covalent Organic Frameworks ....................................................................................... 7

2.3 Covalent Organic Frameworks as Films ....................................................................... 12

2.4 Summary and Implications ........................................................................................... 16

Chapter 3: Research Design .............................................................................. 17

3.1 Experimental Design .................................................................................................... 17

3.2 Materials ....................................................................................................................... 18

3.3 Synthesis Approaches and Tools .................................................................................. 23

3.4 Characterisation of COFs ............................................................................................. 26

3.5 Parameters Explored Throughout Work…...………………………………………….28

Chapter 4: Results .............................................................................................. 29

4.1 Solvothermal Films ....................................................................................................... 29

4.2 Sonication ..................................................................................................................... 36

4.3 Spin-Coating ................................................................................................................. 39

4.4 Thermal Imprinting ....................................................................................................... 42

4.5 A Note on Analytical Techniques ................................................................................. 45

4.6 Conclusions .................................................................................................................. 46

iv SYNTHESIS AND CHARACTERISATION OF COVALENT ORGANIC FRAMEWORKS AS THIN FILMS

Chapter 5: Substrate-Supported Membranes through Thermal Processing 49

5.1 Solvothermal ................................................................................................................ 49

5.2 Thermal Imprinting with Cyclohexanone Solution ...................................................... 55

5.3 Thermal Imprinting in the Presence of Water .............................................................. 58

5.4 Solvothermal Synthesis on Aluminium Foil ................................................................ 58

5.5 Conclusions .................................................................................................................. 61

Chapter 6: Substrate-Supported Membranes through Solvent-Vapour

Annealing 63

6.1 Solvent-Vapour Annealing at Room Temperature ....................................................... 63

6.2 Solvent-Vapour Annealing with Thermal Processing .................................................. 68

6.3 Conclusions .................................................................................................................. 71

Chapter 7: Self-Supporting COF-1 Membranes ............................................. 72

7.1 Synthesis Details .......................................................................................................... 72

7.2 Characterisation of the Freestanding Membranes ........................................................ 73

7.3 Discussion .................................................................................................................... 78

7.4 Conclusions .................................................................................................................. 80

Chapter 8: Conclusions ...................................................................................... 83

Bibliography ............................................................................................................. 88

Appendices .............................................................................................................. 100

Appendix A .......................................................................................................................... 100

Appendix B .......................................................................................................................... 102

Appendix C .......................................................................................................................... 104

Appendix D .......................................................................................................................... 105

Appendix E ……………………………………………………………………………….. 106

SYNTHESIS AND CHARACTERISATION OF COVALENT ORGANIC FRAMEWORKS AS THIN FILMS v

List of Figures

Figure 1. Illustration of the conversion of the precursor molecule BDBA to

COF-1.1 Figure reproduced from Côté et al.1 with permission from

The American Association for the Advancement of Science. ....................... 8

Figure 2. Illustration of the conversion of the precursor molecules BDBA and

HHTP to COF-5. 1 Figure reproduced from Côté et al.1 with

permission from The American Association for the Advancement of

Science. .......................................................................................................... 8

Figure 3. Illustration of the preparation of a COF-1 membrane via the assembly

of exfoliated COF-1 nanosheets. 100 Figure reproduced from Li et al.100

with permission from the American Chemical Society. .............................. 13

Figure 4. ACOF-1 membrane on a porous α-Al₂O₃ support. 102 (a) illustration

of synthesis process; (b) Cross-sectional SEM image of synthesised

membrane. Figure reproduced from Fan et al. 102 with permission from

the Royal Society of Chemistry. .................................................................. 13

Figure 5. Structure of PEBA and SEM image of cross-section of TpPa-1-

nc/PEBA composite membrane synthesised by Schiff base

aldehyde−amine condensation.110 Figure reproduced from Zou et al.110

with permission from the American Chemical Society. .............................. 14

Figure 6. (A) Solution casting of colloid yields a coherent, free-standing COF

film. (B) Optical image of transparent freestanding COF-5 film. (C)

SEM of freestanding film.7 Figure reproduced from Smith et al.7 with

permission from the American Chemical Society. ...................................... 15

Figure 7. Project workflow map. ............................................................................... 17

Figure 8. SEM Images of the precursors (a) 1,4 -benzenediboronic acid

(BDBA) and (b) 2,3,6,7,10,11 -hexahydroxytriphenylene (HHTP)

powders ........................................................................................................ 18

Figure 9. The HOPG substrate in (a) photograph143 and (b) illustration of its

crystal structure143 ........................................................................................ 21

Figure 10. Graphene on Cu foil; (a) photograph of foil substrate,145 (b) Copper

foil morphology on graphene on copper foil imaged using SEM ................ 21

Figure 11. Photographs of (a) glass slide,146 (b) SiO2 on Si wafer,147 and (c)

ceramic crucible148 ....................................................................................... 22

Figure 12. SEM images of (a) Teflon (PTFE) and (b) ceramic filter paper

substrates ...................................................................................................... 22

Figure 13. Photograph of the spin-coater used for deposition of films. ..................... 23

Figure 14. Illustration of setup of the solvothermal synthesis ................................... 24

Figure 15. Illustration of the setup of the room temperature solvent-vapour

synthesis ....................................................................................................... 25

vi SYNTHESIS AND CHARACTERISATION OF COVALENT ORGANIC FRAMEWORKS AS THIN FILMS

Figure 16. Schematic illustration of the thermal imprinter (a), and photograph

of the imprinter (b). ...................................................................................... 26

Figure 17. Crystallite distribution of drop-cast solovothermal samples imaged

usng SEM; (a) HOPG, (b) SiO2/Si wafer, and (c) graphene on Cu foil;

IR spectra in (d) of the post-synthesis crystallites with as-recieved

BDBA powder for comparison. ................................................................... 30

Figure 18. IR spectrum of solvothermally annealed crystallites and precursor

powder…………………………………………………………………......31

Figure 19. COF-1 crystallites imaged using a polarizing light microscope. (a)

sample prepared using heptanoic acid solution; (b) sample prepared

using heptanoic acid-ethanol solution. ......................................................... 32

Figure 20. COF-1 crystallites imaged using SEM. (a) sample prepared using

heptanoic acid solution; (b) sample prepared using heptanoic acid-

ethanol solution. ........................................................................................... 32

Figure 21. Heptanoic acid/ethanol solvothermal BDBA crystals high-resolution

imaged using helium ion microscopy; (a) layered crystal morphology

of the heptanoic acid/ethanol treated BDBA crystal; (b) close-up of

the crystal layers. ......................................................................................... 35

Figure 22. Crystal morphologies of BDBA crystals solvothermally synthesised

in various solvents imaged using SEM; (a) ethanol, (b) 0.5:1

heptanoic acid/ethanol, (c) 1:1 ethanol/acetone, (d) acetone . ..................... 35

Figure 23. Comparing two instances of sonication of graphene on Cu foil in a

vial of solution, with the second experiment using the same vial and

solution as well; (a) film morphology of first experiment; (b) close-

up of sparse crystal distribution; (c) film morphology of second

experiment; (d) close-up of crystal distribution. ........................................ 37

Figure 24. Film on various substrates synthesised via ultrasonication imaged

using SEM; (a) graphene on Cu foil and (b) HOPG. ................................... 38

Figure 25. Other crystal and particle morphologies observed on a post-

sonicated graphene on Cu foil imaged through SEM; (a) blocks of

crystals dispersed randomly on film; (b) crystals appearing to be

broken chunks of a much longer piece; (c) small crystals dispersed

randomly on more uneven regions of the substrates. ................................. 38

Figure 26. BDBA solution spin-coated on two different substrates imaged

using (a) HIM on the Si wafer and (b) SEM on the HOPG. ........................ 40

Figure 27. Comparison of films spin coated and then annealed on two different

substrates. (a-b) film on Si wafer imaged using (a) optical microscopy

and (b) helium ion microscopy. (c-d) film on HOPG imaged using (c)

optical microscopy and (d) scanning electron microscopy. ......................... 41

Figure 28. IR Spectra of thermally annealed spin-coated COF-1 film vs BDBA

precursor powder .......................................................................................... 41

Figure 29. SEM images of morphological variations of thermally imprinted

COF-1 films on Si substrates synthesised on four different days using

the same solution; (a) layering of crystals in various distractions; (b)

crystal mass appearing to be more fused together than in (a) due to

SYNTHESIS AND CHARACTERISATION OF COVALENT ORGANIC FRAMEWORKS AS THIN FILMS

vii

greater impact from compression; (c) dense masses of crystals; (d)

densest packing of crystals seen in all samples. ......................................... 43

Figure 30. Thermally imprinted COF-1 film imaged using a stereomicroscope;

(a) spin-coated substrate, and (b) clean substrate. ....................................... 44

Figure 31. IR spectra of thermally imprinted COF-1 film and BDBA precursor

powder. ......................................................................................................... 44

Figure 32. Other morphologies observed on thermally imprinted COF-1 films

imaged using SEM; (a) leaf-like crystal growth; (b) uneven

distribution of pellet-shaped crystals; (c) sparse masses of crystal

aggregations. ................................................................................................ 45

Figure 33. COF-1 film on a Si wafer synthesised via thermal annealing imaged

using the stereomicroscope for (a) and (b) and SEM for (c); (a) film

morphology post-synthesis; (b) sponge-like underside of film; (c)

close-up of the underside of film. ............................................................... 50

Figure 34. COF-1 film on an HOPG substrate synthesised via thermal

annealing imaged using a stereomicroscope; (a) curled-films post-

synthesis; (b) close-up of films with spherical droplets visible. ................ 50

Figure 35. COF-1 film thermally annealed with distilled cyclohexanone imaged

using a stereomicroscope; (a) film morphology post-synthesis; (b)

sponge-like morphology of the underside of denser, more opaque

films. ............................................................................................................ 51

Figure 36. IR spectrum of COF-1 film obtained by thermal annealing with

BDBA powder for comparison. ................................................................... 51

Figure 37. COF-1 film synthesised on a ceramic crucible via thermal annealing

imaged using SEM; (a) Film morphology post-synthesis; (b) close-up

of crystal growth and fractures. .................................................................. 52

Figure 38. IR spectra of the surface, underside and powder form of the COF-1

film synthesised via thermal annealing on a ceramic crucible with

BDBA powder shown for comparison. ........................................................ 52

Figure 39. Film synthesised on a Teflon filter paper via thermal annealing

imaged using SEM. (a) continuous, smooth region of the film post-

synthesis; (b) porous, sponge-like crystal morphology dominant on

film; (c) cracks appearing to be influenced by the filter web texture;

(d) close-up of the cracks. ........................................................................... 53

Figure 40. IR spectrum of the film formed on teflon filter paper with BDBA

powder data for comparison. ........................................................................ 54

Figure 41. SEM images of BDBA film synthesised via solvothermal annealing

on a ceramic filter paper. (a) film morphology post-synthesis; (b)

close-up of the porous, sponge-like crystal morphology dominant on

the film; (c) partial coverage of film (right) on the filter paper. ................ 55

Figure 42. COF-1 film synthesised via thermal imprinting on an SiO₂ wafer

imaged using stereomicroscope in (a) and the SEM in (b), (c), and (d).

(a) film morphology post-synthesis; (b) spherical masses that make

up the 'holey' appearance in (a); (c) close-up of connected spherical

masses; (d) interwoven crystal growth in the less 'holey' regions. ............ 56

viii SYNTHESIS AND CHARACTERISATION OF COVALENT ORGANIC FRAMEWORKS AS THIN

FILMS

Figure 43. Repeat experiment of COF-1 film synthesised via thermal

imprinting on an Si wafer imaged using stereomicroscope in (a) and

(b) and the SEM in (c), and (d). (a) film morphology post-synthesis;

(b) optical close-up of the droplet-like features; (c) details of the

droplets; (d) a variation of the droplet feature seen on a different

location on the same sample. ...................................................................... 57

Figure 44. IR spectrum of thermal imprinted film using the

cyclohexanone/ethanol/ether solution vs BDBA precursor powder to

show absence of peak shifts or conversion. ................................................. 57

Figure 45. COF-1 film synthesised via thermal imprinting with the presence of

water imaged using SEM. (a) crystal distribution post-synthesis; (b)

close-up of the web-like structure of crystals due to degradation. ............ 58

Figure 46. COF-1 film synthesised via solvothermal annealing on aluminium

foil with refined solution imaged using HIM and SEM. (a) film

morphology of film delaminated from the aluminium foil (in the

background); (b) close-up of flat region seen in (a); (c) layering of

crystals on the delaminated film; (d) fracture behavior of films still

adhered to aluminium foil. .......................................................................... 60

Figure 47. IR spectrum of COF-1 synthesised via solvothermal annealing on

aluminium foil with BDBA powder for reference. ...................................... 60

Figure 48. COF-5 film synthesised via room temperature solvent-vapour

annealing on a glass slide imaged using stereomicroscope in (a) and a

close-up in (b) and the SEM in (c), and a close-up in (d). ........................... 64

Figure 49. COF-5 film synthesised via room temperature solvent-vapour

annealing on a Si wafer imaged using stereomicroscope in (a) and a

close-up in (b) and the SEM in (c), and a close-up in (d) ........................... 65

Figure 50. IR spectrum of COF-5 film synthesised via solvent-vapour annealing and

the COF-5 solution.………………………………………………………..65

Figure 51. Crystal morphology of unsuccessful synthesis of COF-5 film via

room temperature solvent-vapour annealing imaged using imaged

using stereomicroscope in (a) and (b) and the SEM in (c), and (d). (a)

film morphology post-synthesis; (b) close-up of BDBA and HHTP

crystal combination; (c) close-up of crystal growth on edge of film;

(d) general crystal morphology. ….. ........................................................... 66

Figure 52. Film synthesised via room temperature solvent-vapour annealing on

an SiO₂ wafer imaged using stereomicroscope in (a), (b), and (c) and

the SEM in (d). (a) film morphology after synthesis; (b) close-up of

shield like film near the edge of substrate; (c) crystal morphology if

'shield' was absent; (d) close-up of the maze-like structure in (a). ............ 67

Figure 53. IR spectrum of film synthesised via solvent-vapour annealing at

room temperature with BDBA precursor powder for comparison............... 68

Figure 54. IR spectrum of the COF-1 film synthesised via thermal solvent-

vapour annealing with a BDBA powder precursor spectrum for

comparison. .................................................................................................. 69


ix

Figure 55. Surface distribution of COF-1 film synthesised on Si wafer via

thermal solvent-vapour annealing imaged using an (a) optical

microscope, and (b) polarizing microscope; Crystal (c) distribution

and (d) morphology imaged at high-resolution using SEM. ........................ 70

Figure 56. XRD spectra of COF-1 film synthesised on a Si wafer via thermal

solvent-vapour annealing. ............................................................................ 70

Figure 57. COF-1 self-supporting membranes synthesised via thermal

annealing and vapour annealing. (a) cross-section of bottom layer of

both membranes; (b) surface bottom layer of solvothermally annealed

membrane; (c) cross-section of top and bottom layer of vapour

annealed membrane; (d) surface of bottom layer of the vapour

annealed membrane; (e) surface of the top bottom layer of vapour

annealed; (f) stereo-optical image of surface of top layer of

solvothermally annealed membrane. ........................................................... 74

Figure 58. AFM phase images and roughness profile of (a) solvothermally

annealed COF-1 and (b) vapour annealed COF-1 membranes .................... 75

Figure 59. XRD spectra of solvothermally annealed (blue) membrane with a

sharper peak and less evidence of unreacted BDBA than the vapour

annealed (green) membrane. ........................................................................ 76

Figure 60. Clean Ar gas adsorption/desorption isotherms of TA (blue) and

vapour annealed (green) membranes. .......................................................... 77

Figure 61. Pre-degas (darker shade) and post-degas (lighter) FT-IR spectra of

TA (blue) and vapour annealed (green) membranes. ................................... 77

Figure 62. Nanoindentation loading curves for (a) TA COF-1 and (b) vapour

annealed COF-1 membranes with measurement spacing matrix in

onset. ............................................................................................................ 78

Figure 63. Setup of ultrasonic vibration assisted drop-casting. .............................. 100

Figure 64. Comparison of a COF film synthesised via room temperature

solvent-vapour annealing with solution drop-casted (a) and (b) with

assistance from ultrasonic vibrations and (c) and (d) without. .................. 101

Figure 65. Setup of COF-1 bulk powder synthesis via solvothermal processes. ..... 102

Figure 66. IR spectra of bulk powders synthesised via Côté’s1 procedure. ............. 103

Figure 67. Film and crystal morphology of COF-1 film synthesised via partial

solvothermal annealing and then plasma treatment. (a) film

morphology after plasma-treatment; (b) colour variations of crystals;

(c) white specks appearing to outline crystal shapes; (d) dendritic-

like darkening of crystals. ......................................................................... 104

Figure 68. Other crystal morphologies observed on a COF-1 film synthesised

via solvent-vapour annealing on a Si wafer. (a) crystal morphology

of overall film; (b) crystal morphology of the more homogenous

regions; (c) close-up of pointed edges of (b); (d) radiating

agglomerates of spindle-like crystals; (e) layered, intergrowth of

shell-like structures; (f) circular-disk shaped agglomerates of

crystals. ...................................................................................................... 105

x SYNTHESIS AND CHARACTERISATION OF COVALENT ORGANIC FRAMEWORKS AS THIN FILMS

List of Tables

Table 1: Key Achievements in the Study of COFs .................................................... 10

Table 2: BDBA Solubility Table ................................................................................ 19

Table 3: Micropore-Mesopore Ratio of Surface Areas……………………………...79

Table 4: Syntheses in Chronological Order ............................................................... 84


xi

List of Abbreviations

COF Covalent organic frameworks

BDBA 1,4 -benzenediboronic acid / phenylenediboronic acid

HHTP 2,4,6,7,10,11 -hexahydroxytriphenylene

MOF Metal organic frameworks

COF-1 Covalent organic frameworks-1 (First COF created1)

COF-5 Covalent organic frameworks-5

PEBA Polyether block amide

VFET Vertical field-effect transistor

TTF Tetrathiafulvalene

PDBA Phenylenediboronic acid

NiPc Nickel phthalocyanine

BTDA Benzothiadiazole

PEDOT Poly(3,4-ethylenedioxythiophene)

DTPA Diethylenetriaminepentaacetic acid

FT-IR Fourier transform infrared spectroscopy

SEM Scanning electron microscope

HIM Helium ion microscope

XRD X-ray diffraction

HOPG Highly ordered pyrolytic graphite

SVA Solvent vapour annealing

AFM Atomic force microscopy

IR Infrared spectroscopy

TEM Transmission electron microscopy

BET Brunauer-Emmett-Teller

DFT Density functional theory

xii SYNTHESIS AND CHARACTERISATION OF COVALENT ORGANIC FRAMEWORKS AS THIN

FILMS

Statement of Original Authorship

The work contained in this thesis has not been previously submitted to meet

requirements for an award at this or any other higher education institution. To the best

of my knowledge and belief, the thesis contains no material previously published or

written by another person except where due reference is made.

Signature:

Date: May 2019

QUT Verified Signature


xiii

Dedication

To Nicole.

xiv SYNTHESIS AND CHARACTERISATION OF COVALENT ORGANIC FRAMEWORKS AS THIN

FILMS


xv

Acknowledgements

I would like to thank my principal supervisor for her passion, enthusiasm and

endless guidance from the moment we first met back in my undergraduate years. Dr

Jennifer MacLeoad is without a doubt the reason writing the thesis, was a joy every

step of the way.

Thank you to my associate supervisor, Professor Steven Bottle for his generous

advices and constant reminder that chemists and physicists don't function the same

way, at all.

Thank you to my second associate supervisor, Dr Llewellyn Rintoul, for his

candor, philosophies on academia and memorable one-liners. And also for his

expertise like no other in IR, but that has always been part and parcel with Llew. His

no-nonsense attitude made me a better academic and I cannot be more grateful for

that.

Thank you to the CARF team for helping me monumentally with obtaining the

best data for my samples. They are what moves my project forward.

And last but not least, to the SEF and CPME HDR team. Thank you for

making my Masters journey a wonderful one and my PhD another endeavor I am

very much looking forward to begin.

Chapter 1: Introduction 1

Chapter 1: Introduction

1.1 BACKGROUND

Covalent organic frameworks (COF) are covalently-bonded organic materials

comprising the light elements boron, hydrogen, carbon and oxygen with high permanent

porosity due to their nanoporous structure. Many COFs are formed as 3D bulk crystals of

2D sheets, analogous to graphite/graphene. They were first synthesised by condensation

reactions of 1,4-benzenediboronic acid (BDBA) alone and with 2,3,6,7,10,11-

hexahydroxytriphenylene in 2005.1 In this synthesis, a closed reaction system was used to

maintain the presence of H2O, an essential factor to facilitating reversible bonding

conditions conducive to high-quality crystallite growth. The reaction progresses through

the formation of a boroxine ring and elimination of three water molecules and can be

reversed through the presence of excess water. Maintaining the presence of H2O is an

essential factor to facilitating the reversible conditions favourable to crystallite growth.2,3

The success of COF synthesis is based on overcoming the crystallization problem

in covalently bonded solids, wherein irreversible covalent bonds prevent error

correction.1,4 This can be avoided by balancing the relevant kinetics and thermodynamics

to promote reversibility of the bond formation, the key factor for extended crystalline

structures. Furthermore, the tunability of the chemical and physical properties of the

building-block molecules permits, in principle, controllable configuration of the material

structure.

Performing the condensation reaction in the presence of a graphene substrate leads

to staggered planar two dimensional sheets, roughly oriented with their basal planes

parallel to the surface, improving crystallinity compared to COF powders.5 In monolayer,

surface-supported form, COFs can be used in a range of applications. For example, they

can be used to template molecules through selective adsorption of molecules. This

characteristic can be utilised in sensing applications by maximizing the exposed active

area, and by increasing the thickness of the COFs from monolayer to multilayer.2

2 Chapter 1: Introduction

Here, we demonstrate a variety of approaches to creating COF films, both substrate-

bound and self-supporting. These methods provide a framework for easy tailoring of the

thickness, crystal size and pore size of the membranes. We characterize these films using

microscopy, x-ray diffraction, infrared spectroscopy, atomic force microscopy, and

nanoindentation. This type of understanding lays the groundwork for optimization of these

2D membranes, and their inclusion in applications, for example gas storage and filtration.

1.2 CONTEXT

Recent attention has turned to the synthesis of COFs beyond their original bulk

powder form and into nanosheets, composites, coating, gels, and membranes, targeting

applications such as molecular separations.6,2,7-10 This ability to synthesise freestanding

COF membranes brings us closer to applications where the COFs can be integrated into

engineered systems for, e.g., flow-through filtration applications. However, moving to

these applications requires the development of an understanding of the chemical, physical

and mechanical properties of these COF membranes, which are key to their successful use

in applications, particularly where the films may be required to self-support.

1.3 PURPOSE

The purpose of this study is to develop replicable and scalable (from nm to mm)

techniques for creating covalent organic framework thin films, both substrate-bound

and self-supporting.

1.4 SIGNIFICANCE, SCOPE AND DEFINITIONS

This project reports on the fabrication of COF beyond its original bulk powder

form, the challenges inherent in forming COF films, and implications for future

research in this field.

1.4.1 Thin Films and Membranes

The terms films, thin films, and membranes will be used interchangeably

throughout the thesis.

1.4.2 Silicon Wafer

Although the silicon wafer comprises a thin SiO2 native oxide layer on its

surface, the wafer will be defined simply as an Si wafer.

Chapter 1: Introduction 3

1.4.3 Room Temperature Solvent-Vapour Annealing

The terms solvent-vapour annealing, and vapour annealing will be used

interchangeably and is to mean vapour-assisted synthesis at room temperature. Vapour

annealing at other temperatures will be specified explicitly.

1.4.4 I/We

Use of “we” is a stylistic preference and refers to the single author of the thesis.

1.4.5 Scalable

The term scalable throughout the thesis unless otherwise stated is defined as

increasing the lateral length scale of the product film from the range of nm to

mm.

1.5 THESIS OUTLINE

Chapter 2 presents a literature review of covalent organic frameworks in their

many variations and forms, especially as thin films.

Chapter 3 details the experimental aspects of the project such as the materials,

experimental techniques and characterisation instruments used.

Chapter 4 describes the pilot studies undertaken to replicate bulk COF synthesis

techniques reported in the literature, and to characterize the resulting product. Can we

successfully synthesise a COF-1 film with the chosen methods? What parameters have

had the most influence?

Chapter 5 reports results on substrate-supported membranes synthesised through

thermal processing. Can we synthesise a mm scale COF-1 film through solvothermal

annealing? How does the solvent system chosen affect film growth and morphology?

Chapter 6 reports results on substrate-supported membranes synthesised through

solvent-vapour processing. Can we synthesise a mm scale COF-1 film through solvent

vapor annealing? How does substrate choice affect film growth and morphology?

Chapter 7 reports on the synthesis of self-supporting COF-1 membranes. What

were the resulting chemical, physical and mechanical properties of samples

synthesized via solvothermal annealing and solvent vapor annealing?

Chapter 8 closes the thesis and provides a brief deliberation on directions for

future work.

Chapter 2: Literature Review 5

Chapter 2: Literature Review

2.1 WHAT MAKES A POROUS MATERIAL?

The demands of society more often than not influence the development of

scientific knowledge and the functional materials imagined from this knowledge. The

concept of a porous material is one of them. Documented scientific research on

charcoal, a material dating back to the Ancient Egyptians,11 was not evident until the

18th century with the investigation of its adsorptive capabilities by Scheele, Priestley

and Fontana.12,13 At a similar time, Cronstedt14 discovered the frothing nature of a

stilbite he heated rapidly, naming the resulting mineral zeolite. Zeolite, a naturally

occurring and industrially produced mineral, is a common example of a porous

material/mineral that has been extensively studied due to its excellent physical and

chemical properties. Low production costs have made it an attractive material for a

wide range of applications, such as CO2 capture, water purification systems, and

oxygen concentrators.15-17 However, zeolites are susceptible to decreased performance

under humid conditions, are difficult to intrinsically fine-tune and functionalize, and

have low pressure capacity due to their small pore volume, posing a great obstacle to

widespread applications.18

Hence, interest in other types of engineered porous structures has grown in the

past decade due to their potential application in gas storage, nanotemplates and high

performance components of electronic devices.19,20 Organic thin films with porous

molecular structures have applications in technologies such as organic light emitting

diodes, to aid in maximising light emission through optical scattering21,22 and field-

effect transistors, for more efficient pathways for diffusion of gas molecules through

channels and hence improving overall performances such as on sensors.23,24 These

devices can potentially enable future low-cost electronics with uncompromised

performance, as they can be fabricated on cheap substrates such as glass and plastic,

and do not require capital-intensive manufacturing plants that are required for silicon-

based products. They can be fabricated using simple techniques such as screen-

printing, and spin-coating.25,26 Apart from cost, another important factor of increasing

interest is scalability. The advancement of technologies will depend on the shrinking

of micro-scale devices such as transistors (a 'top-down' approach), or building

6 Chapter 2: Literature Review

transistors molecule-by-molecule ('bottom- up'). Established microfabrication

techniques such as focused ion beam lithography or sputtering are top-down

approaches, and hence face physical limitations as they reach the molecular scale.27

Self-assembling molecules can overcome this problem as they can be incorporated into

bottom-up developments of nanostuctures. Self-assembly is based on molecules that

facilitate reversible non-covalent intermolecular bonds, resulting in long range ordered

structures.28 These bonds, however, are known to be weak and are easily

compromised, especially at increased temperature. The formation of covalent bonds

can overcome this problem, granting mechanically stable networked structures.29

The importance of ordering and control at the molecular level is also apparent in

other applications of organic films, such as electronics. The performance and

efficiency of electronic devices dramatically depend on the quality of the interface

between the electrodes and small-molecule molecular active layers that control the

carrier injection, a mechanism crucial to the fidelity of semiconductors in solid-state

electronics.30 Crystalline organic materials also have superior transport properties due

to their higher degree of molecular ordering.31 Increasing ordering in molecular films

is a key experimental challenge. Some of the ways include tuning of solvent system to

improve solvent-molecule interactions,32 or templating, where molecules adopt

positions consistent with the ordering on the template.33 A novel approach to this

problem has recently been reported: thermal imprinting is a form of nano imprint

lithography that involves the depositing a thin layer of solution in the substrate via

spin-coating and then brought into contact with a clean substrate and pressed together

with a calibrated pressure.34 This method was recently used to synthesise a

subphthalocyanine film with stable highly crystalline domains.35

Similar challenges exist for polymer films. Conductive polymers are widely used

in technologies such as solar cells, nano-fibers, liquid crystal displays and electrode

coatings due to their high stability, tunable electrical properties and low redox

potential. Conductive polymers, however, can have limited processibility,36,37

resulting in inhomogeneity in their films. The varying chain lengths, defects and chain

ends on these polymers contribute to the irregularity. Additionally, the chains can

orient in all three axes (x, y, z), giving each chain differing electronic properties. This

disorder leads to the localization of charges, affecting the overall electronic properties

of the film.38 Establishing deposition and synthesis conditions that allow for better


control of the morphology and crystallinity is of great interest.

The search for materials that overcome the limitations inherent in zeolites, and

that may lead to the ordering necessary for incorporation of organic materials in a

range of applications led to the discoveries of metal-organic frameworks (MOFs) and

covalent organic frameworks (COFs), two prominent classes of porous crystalline

assemblies with high gas capture capacity and selectivity, good stability and

reusability, uniform pore sizes and low energy requirement for regeneration.18 COFs

are the material of interest in this thesis.

2.2 COVALENT ORGANIC FRAMEWORKS

Organic frameworks with covalent bonds can be obtained from reversible and

irreversible reactions. Irreversibility, however, leads to poor crystallinity or randomly

placed structures as there is no mechanism for correcting defects. This means that, in

most cases, long range order crystallinity cannot be achieved. Covalent organic

frameworks, on the other hand, are formed from reversible bonding processes,

allowing for error-checking and reduction or elimination of structural flaws until a

stable state is established.1,20 These organic materials are composed of building blocks

made from light elements such as boron, hydrogen, carbon, and oxygen and have high

permanent porosity due to their nanoporous structure. COFs can be formed as 3D

crystals comprising 2D sheets, analogous to graphite/graphene. The first COF, known

as COF-1, was synthesised by Yaghi and his team in 2005 through a condensation

reaction of the 1,4-benzenediboronic acid monomer, converging the boronic acid

molecules and eliminating water molecules to form a planar six-membered boroxine

ring (Figure 1). Yaghi and coworkers also demonstrated a second COF, COF-5, which

has two monomers, 1,4-benzendiboronic acid and hexahydroxytriphenylene, and is

formed through the dehydration reaction of the boronic acids and diols, resulting in a

five-membered borate ester ring as illustrated in Figure 2.8 A closed reaction system

was in place for both syntheses. This system maintains the presence of H2O, an

essential factor to facilitating reversible conditions conducive to high-quality

crystallite growth. In 2011, Colson et al.39 found that performing the condensation

reaction in the presence of a graphene substrate still led to the formation of the

boroxine ring but also resulted in staggered, planar two dimensional sheets roughly

oriented with their basal planes parallel to the surface, improving crystallinity


compared to the COF powders produced by Côté et al.1

Because the backbone of a COF constitutes a periodic network, the design of

new COFs may start with determining the desired dimensionality of these systems,

either 2D or 3D, with research on both types of COF equally vast.40 Networks

extending in two dimensions are ultimately restricted to planar covalent bonds, and

generally have π-π stacking providing attractive intermolecular interaction between

the layered planes.41,42 A 3D network builds in all directions, resulting in a more

isotropic morphology, with the most common geometric net adopted for this structure

being the tetrahedron.43,44

Figure 2: Illustration of the conversion of the precursor molecules BDBA and HHTP to COF-5.

Figure reproduced from Côté et al. 1 with permission from The American Association for the

Advancement of Science.

Figure 1: Illustration of the conversion of the precursor molecule BDBA to COF-1. Figure

reproduced from Côté et al.1 with permission from The American Association for the

Advancement of Science.


Considering that COFs comprise both linkages (bonding nodes) and struts

(organic spacers)20 the innovations in COF design can be broadly categorised.

Modification of the chemistry at the nodes allows control of the geometry, and has

been approached through reactions based on hydrazone,45-47 azine,48-51 triazine, based

on dynamic trimerization52,53 and more prominently, imine, which is generally based

on Schiff-based reactions.54-62 Modifications to the organic struts can add

functionality: the addition of a phthalocyanine63-67 introduces metal centers which can

enforce planarity and encourage an eclipsed stacking geometry that allows electron

delocalisation, facilitating charge carrier transport within the COF.64,65,67 Similarly,

porphyrins can be used to modify the band gap and enhance photocatalytic activity.68-

73 Tetrathiafulvalene (TTF), an organosulfur compound, has been integrated into a

porphyrin-based COF, boosting carrier transport and enhancing electric conduction.74

In monolayer, surface-supported form, COFs can also be used to template molecules,

and through selectively adsorption of molecules from solution. This characteristic can

be utilised in sensing applications by maximizing the exposed active area, and by

increasing the thickness of the COFs from monolayer to multilayer.12,13

The following is a summary, in chronological order, of the key achievements

that have been made since the birth of this class of materials. While there is an

abundance of evidence for rational synthesis through functionalization and geometry

control of organic molecules, strategies to fully control crystallite-scale morphology

and crystallographic orientation in films are still being established.

Table 1: Key Achievements in the Study of COFs

Year Achievement

2005 Synthesis of the first COFs, COF-1 and COF-5, via condensation reactions of

BDBA and HHTP.1

2006 Co-condensation of a linear alcohol with a triboronic acid (COF-18).75

2007 Extension of 2D hexagonal COFs by linking trigonal molecules, giving COF with

pore sizes between 9 and 32 (COF-6, COF-8, and COF-10).76

2007 Condensation of the zinc porphyrin tetraboronic acid with tetrahydroxybenzene,

producing COF-108, one of the most porous COFs with the lowest density.77


2008 Condensation of borosilicate clusters (Pyrex) producing thermally and chemically

stable 3D COFs.78

2009 Utilisation of electroactive organosilane precursors with a surfactant-templated

synthesis to create porous hole-conducting framework material.79

2009 Synthesis of highly ordered conjugated pyrene COF using the co-condensation

reaction of triphenylene HHTP and pyrenediboronic acid (PDBA), giving a

hexagonal framework with eclipsed arrangements.80

2010 Reaction of tetragonal phthalocyaninetetra(acetonide) with a linear diboronic acid

and Lewis acid to form a tetragonal lattice.67

2011 A metallophthalocyanine COF and a porphyrin-based COF was synthesised,

creating a photoconductive COF which exhibits high charge carrier mobilities and

broad absorption profile.65

2011 Dehydration reactions produced hydrazone linked COFs.45

2011 COF with accessible pores of 4 nm was achieved.81

2011 Condensation reaction of porphyrin via a boronate ester formation (COF-66) or

imine bond formation (COF-366), producing macrocyclic COFs exhibiting

superior semiconductor properties.73

2011 Electron-transporting COF synthesised through substitution of benzene groups in

NiPc-COF electron deficient benzothiadiazole (BTDA) units.64

2012 Functionalization of 3D COFs using a monomer-truncation strategy.82

2013 Squaraine-based COF was produced.72

2014 Covalent TTF lattice through integration of TTF units to form 2D COFs.74

2014 Vapour-assisted synthesis of COF without a substrate, resulting in a nanofibrous

morphology.83

2015 An azine-linked COF was synthesised under solvothermal condition.50


2015 Room-temperature vapour-assisted synthesis of COF-5, resulting in films < 10

microns thick.84

2016 Through imine-condensation reactions, a 3D COF with the ability to mutually

weave periodically was produced from helical organic threads.85

2016 Proposed condensation systems that utilises one knot and multiple linker units to

synthesise 2D and 3D multiple-component COFs.86

2016 Proposed the utilisation of aggregation-induced emission mechanism to create

highly emissive COFs.87

2016 The growth of the poly(3,4-ethylenedioxythiophene (PEDOT) conductive polymer

inside the pores of the COF to enhance transport and electrical capacities.88

2017 Stable colloidal suspensions of 2D COF particles were produced under

homogeneous 5polymerization conditions that prevented crystallite precipitation.6

2017 Growth of full-conjugated 2D COF on a dielectric hexagonal substrate.30

2017 Synthesis of ultraporous COF of various shapes through an ancient terracotta

process was demonstrated.57

2018 Layer-by-layer synthesis of imine-linked COF on a porous ultrafiltration membrane

with tunable thickness and unprecedented water permeance.89

2018 Interfacial polymerization of flexible COF thin films that possess exceptional

mechanical strength and durability in high humidity environment.90

2018 Phase transformation from compressive loading of COF grown on a porous DTPA

sheet observed through simulation models.91

2018 Study on the effect of water adsorption on CO₂ capture on COFs using monte carlo

simulations.92

2018 Etched-off copper assisted thermal conversion of COF capsules for metal-free

electrode materials.93

2018 Oil-confined interfacial synthesis of transferable COF films.94


2.3 COVALENT ORGANIC FRAMEWORKS AS FILMS

Organic compounds have a prominent place in modern material science, whether

in fundamental research or commercialization.95 Natural abundance of precursors

(e.g., carbon) and low production cost are just some of the advantages over their

inorganic counterparts. Hence it may be no surprise that the majority of membranes

manufactured for research and commercial use are organic polymers.96-98 In addition

to the aforementioned advantages, they are easy to prepare and can be tailored for

specific functions. These membranes, however, cannot compete with their inorganic

or metallic counterparts when it comes to resistance to high temperature and harsh

chemicals. COFs synthesised in film form may be able to overcome these obstacles.

COF films can be directly grown on -Al2O3 ceramic substrates. The first two

COFs synthesised, COF-1 and COF-5, were successfully grown as nanosheets and

micron scale membranes respectively on these ceramic supports.99,100 The exfoliation

method used is presented in Figure 3. Other variants of COFs, both 2D9,101,102 and

3D103 have also been synthesised on -Al2O3 substrates and tested for gas separation

and capture with excellent long-term stability. An illustration of the synthesis of

ACOF-1 is given in Figure 4(a) and the synthesised product in Figure 4(b).101 When

applied as a coating layer on a ceramic separator, Wang et al.104 found that a COF

produced dramatically improved cycling performance of a lithium-sulfur battery.

Figure 3. Illustration of the preparation of a COF-1 membrane via the assembly of exfoliated COF-1

nanosheets. Figure reproduced from Li et al. 100 with permission from the American Chemical Society.


Other substrates have also been demonstrated as suitable supports for COFs.

Medina et al.84 employed a room temperature solvent-vapour technique to grow COF-

5 on SiO2 substrates, resulting in micron-scale films. Another type of COF, a hybrid

of COF and metal-organic framework (MOF), was modified with polyaniline and

grown on SiO2 for potential H2/CO2 separation.105 COFs can also be synthesised on

fibrous substrates such as filter paper or soft cloth.106 Colson et al.39 grew COF-5 on

single-layer graphene substrates, producing oriented films ~75 nm thick. More

recently, an imine-linked COF was used as the transport channel layer on top of the

single-layer graphene source electrode in a vertical field-effect transistor (VFET),

giving exceptional ambipolar charge transport behaviour due to improved crystallinity

and controllable orientation.107

In addition to films supported on substrates, COFs have been integrated into

mixed matrix membranes, which are made of homogenously harmonized polymeric

and/or inorganic particle matrices.108 The incorporation of COFs as one of the

polymeric components results in a membrane with exceptional gas separation,108-116

liquid separation,116,117 desalination118 and intrinsic proton conduction119 capabilities.

A polyether block amide (PEBA)-based COF nanosheet synthesised via Schiff-based

condensation is shown in Figure 5. Other interesting membrane structured materials

that have been functionalised with COFs include tyrene-butadiene rubber,120

Figure 4. ACOF-1 membrane on a porous α-Al₂O₃ support. (a) illustration of synthesis process; (b)

Cross-sectional SEM image of synthesised membrane. Figure reproduced from Fan et al. 101 with

permission from the Royal Society of Chemistry.

(a)

(b)


polybenzimidazole,121 membrane electrode assembly pellets,122 alumina tubes123 and

elastomers.124

While the benefits of integrating COFs into multicomponent membranes are

evidently numerous, there is also great interest in freestanding COF membranes.

Unlike the substrate-bounded alternative, self-supported membranes are transferrable

and can be integrated into various geometries.

A number of studies have demonstrated the solution-based synthesis or

exfoliative isolation of nanosheets.6,125-133 The colloid-based synthesis of freestanding

COF-5 nanosheets is illustrated in Figure 6. There are however, limitations to

exfoliation such as control of the thickness, limitations to sheet size and presence of

residual surfactants or solvents adsorbed into the sheets.134,135 However, work on

micron scale self-supporting COF membranes is has only emerged in recent

years. These membranes have the advantage of withstanding harsh chemicals and,

depending on the synthesis technique, can be scalable. Sasmal, et al.136 fabricated a

self-standing flexible COF membrane that facilitates superprotonic conductivity. A

palladium-loaded COF deposited into PTFE molds and then irradiated with UV light

produced freestanding membranes that proved to be functional for room temperature

chlorobenzene dechlorination in water137.

Figure 5. Structure of PEBA and SEM image of cross-section of TpPa-1-nc/PEBA composite membrane

synthesised by Schiff base aldehyde−amine condensation. Figure reproduced from Zou et al. 110 with

permission from the American Chemical Society.


This ability to synthesise freestanding COF membranes with thicknesses on the

micron scale, brings us closer to applications where the COFs can be integrated into

engineered systems for, e.g., flow-through filtration applications. However, moving to

these applications also requires the development of an understanding of the mechanical

properties of these COF membranes, which are key to their successful use in

applications, particularly where the films may be required to self-support.

2.4 SUMMARY AND IMPLICATIONS

When combined with their robustness, porosity and potentially excellent

electronic properties, crystallographic control in COFs is a key ingredient to both

innovative and impactful nano-applications. However, there is still much to gain in our

understanding of the basic mechanisms controlling the assembly of covalent organic

frameworks. This knowledge is crucial to further progress the development of

techniques that will ultimately lead to covalent organic frameworks that are

crystallographically and structurally well-defined in film form, whether substrate-

confined or freestanding. When control and tunability of the properties of these films

is achieved, it will, in principle, pave the way to new functionalities and applications.

Figure 6. (A) Solution casting of colloid yields a coherent, freestanding COF film. (B) Optical image

of transparent freestanding COF-5 film. (C) SEM of freestanding film. Figure reproduced from

Smith et al.6 with permission from the American Chemical Society.

Chapter 3: Research Design 17

Chapter 3: Research Design

3.1 EXPERIMENTAL DESIGN

Figure 7 details the typical experimental workflow of this project. We began with a

survey of published work for techniques that could be adapted to synthesizing COF-1 on

a substrate. Suitable strategies were selected and enacted. Once synthesised, each film

was then analysed using infrared spectroscopy to determine if conversion of the precursor

to COF-1 was successful. The experiment was deemed successful if conversion was

Determine crucial

parameters

Translate protocols

to produce self-

supported

membranes

Morphological

(SEM, HIM,

optical)

Chemical (FT-IR) Physical (XRD,

gas adsorption,

nanoindentation)

Adapt to convert

precursor to COF-1

substrate-confined

film

Identify/Refine

techniques from

published work

Figure 7. Project workflow map.

18 Chapter 3: Research Design

observed through shifts and attenuations of the boronic acid-related bands (B-O for

boronate ester, B₃O₃ for boroxine anhydride, and C-O for boroxoles); these bands will be

identified in IR spectra throughout the thesis. We could also roughly estimate COF-1

concentration through the ratio between the BDBA precursor bands and the evolved COF-

1 bands. XRD spectra can be similarly analysed, and ideally can provide quantitative

estimates not possible from IR spectra. The following chapters will discuss these spectral

data in further detail.

Whether an experiment was successful or not, analysis on film morphology was

performed for record keeping. We then returned to identifying a different technique or

refining the technique for further experiments.

Following a successful synthesis, and after morphology analysis, we examined the

film further for its physical properties through XRD, gas adsorption and nanoindentation

where possible. We then analysed the collected data and determined the crucial parameters

for successful translation of the protocol to the synthesis of self-supporting membranes.

3.2 MATERIALS

3.2.1 Chemicals

In this work, we chose to focus almost exclusively on the well-established COF

known as COF-1 as it was the pioneering COF, and is constituted by a single monomer,

1,4-benzenediboronic acid. We additionally made some studies on COF-5, which contains

two monomers, to follow exactly the synthesis strategies described in the literature for

(b)

(a)

Figure 8. SEM Images of the precursors (a) 1,4 -benzenediboronic acid (BDBA) and (b) 2,3,6,7,10,11 -

hexahydroxytriphenylene (HHTP) powders

10 µm

10 µm


solvent-vapour annealed films. The monomers, 1,4-benzenediboronic acid (BDBA, 95%)

and 2,3,6,7,10,11 -hexahydroxytriphenylene (HHTP, 95%) were purchased from Sigma

Aldrich and Tokyo Chemicals Industry, respectively, and were used as received. Figure 8

shows scanning electron microscopy (SEM) images of the precursor molecular powders.

COF synthesis involves dissolution of precursor molecules into solvent, and one of

our objectives in this work was to investigate and optimize solvents and solvent mixtures.

Ethanol (99.5%) and diethyl ether (99%) were purchased from Univar, cyclohexanone

(99%) from M&B Chemicals, cyclopentanone (99%) from ACROS Organics and

heptanoic acid (96%) from Sigma Aldrich. Except for the cyclohexanone, which was

distilled and then filtered, the solvents were used as received.

3.2.2 Solubility of precursors

A basic solubility test was completed by dissolving 10mg of BDBA in 1.5 mL of

solvent and is presented in Table 2. This investigation allowed us to formulate new solvent

systems that helps with the uniform deposition and synthesis of the precursor on the

chosen substrate.

Table 2: BDBA Solubility Table

Solvent

Solubility

of 10mg

BDBA

Appearance

Boiling

Point

(oC) 138-

140

Density

(g/mL)

138-140

Vapour

Pressure

20oC

(hPa) 138-

140

Viscosity

10¯³ Pa

s138-140

Formula1

38-140

1-methyl-2-

pyrrolidinone Complete

Powder

dissolved

easily

202 1.028 32 1.67 C₅H₉NO

2-butanone Mildly Cloudy 79.6 0.805 105 0.41 C4H8O

2-methoxyethanol Complete

Powder

dissolved

easily

124 0.965 6 1.72 C₃H₈O₂

Acetone Mildly Cloudy 56.2 0.786 240 0.3 C3H6O

Acetonitrile Mildly Cloudy 81.6 0.786 97 0.34 C2H3N

Acetyl acetone Mildly

Complete

after

sonication

140.4 0.975 3 - C5H8O2

Anisole Mildly Cloudy 153.7 0.996 3.5 - C7H8O

Chloroform Mildly

Powder

remains on

surface of

solvent

undissolved

61.2 1.498 210 0.54 CHCl3

Cyclohexanone Mildly Clumpy 155.6 0.948 5 2 C6H10O

Cyclopentanone Mildly

Complete

after

sonication

130.6 0.95 11.5 1.29 C5H8O


Dichloromethane Mildly

Some powder

remains on

surface of

solvent

undissolved

39.8 1.326 475 0.42 CH2Cl2

Diethyl ether Insoluble

Solvent stays

clear with no

powder

dissolved

34.6 0.713 587 0.22 C4H10O

Dimethyl sulfoxide Complete

Powder

dissolved

easily

189 1.092 0.61 2 C2H6OS

Dimethylacetamide Complete

Powder

dissolved

easily

165 0.937 300 0.945 C₄H₄NO

Dimethylformamide Complete

Powder

dissolved

easily

153 0.944 3.5 0.8 C₃H₇NO

Dioxane Complete

Powder

dissolved

easily

101.1 1.033 41 1.18 C4H8O2

Ethanol Complete

Powder

dissolved

easily

78.5 0.789 59 1.08 C2H6O

Ethyl acetate Mildly Cloudy 77 0.894 97 0.43 C4H8O2

Ethylene glycol Mildly Cloudy 197 1.115 0.092 16.1 C2H6O2

Heptanoic acid Mildly Cloudy 223 0.912 1.35 5 C7H14O2

Hexane fraction Mildly Clumpy 69 0.655 160 0.29 C6H14

Isopropanol Mildly

Complete

after

sonication

82.4 0.785 44 2.07 C₃H₈O

Methanol Complete

Powder

dissolved

easily

64.6 0.791 128 0.54 CH4O

Tetrahydrofuran Complete

Powder

dissolved

easily

66 0.886 200 0.46 C4H8O

Toluene Mildly Cloudy 110.6 0.867 29 0.55 C7H8

Trichlorobenzene Insoluble

Solvent stays

clear with no

powder

dissolved

213 1.46 1 0.562 C6H3Cl3

p-Xylene Mildly Cloudy 138.3 0.861 15 0.65 C8H10

Effects of concentration

To investigate the effect of monomer concentrations on distribution and

homogeneity on the substrate, we varied the solution concentrations in different trials,

ranging between 0.004 M to 0.22 M. This will be discussed throughout the thesis.

3.2.3 Substrates

The choice of substrate has a profound effect on growth and molecular ordering of

the film, allowing for different film functionalities and applications when selected

carefully.141


Crystalline substrates

Using crystalline substrates creates the platform for ordered epitaxial growth and

through control of the adsorption geometry of precursors. These effects contributed to the

success that Colson et al. achieved in growing ordered COF films.39,63 The highly oriented

pyrolytic graphite (HOPG) is a synthetic graphite that is highly ordered.142 The HOPG we

used was obtained from NT-MDT. The graphene on Cu foil is an alternative graphite-

based substrate that is less brittle and more flexible than the HOPG. The copper foil also

has a texture (Figure 10) that we have found to not have significant effect on the final

synthesis product.

20 µm

Figure 9. The HOPG substrate in (a) photograph143 and (b) illustration of its crystal structure.144

(a)

(b)

Figure 10. Graphene on Cu foil; (a) photograph of foil substrate, 145 (b) Copper foil morphology on

graphene on copper foil imaged using SEM

20 µm

(a)

(b)


Amorphous rigid substrates

Amorphous solids are non-crystalline materials without an ordered lattice pattern.

Despite this irregularity, they are flat, cheap and suitable for the synthesis of our films.

The glass slides were purchased from Sail Brand, the SiO2 on Si wafer from Ted Pella and

the ceramic crucible from ProSciTech (Figure 11).

Amorphous flexible substrates

Aluminium foil has the advantage of low cost, flexibility and scalability. It also

allows for easy removal of films. The aluminium foil used in this work is from Oso.

Filter materials

Filter paper provide for a substrate that is porous and unlike the others above. It is

interesting to see the effect of the porosity on the porous COF. The Teflon filter paper

(Figure 12(a)) was purchased from Advantec and the ceramic filter paper (Figure 12(b))

from Whatman.

Figure 12. SEM images of (a) Teflon (PTFE) and (b) ceramic filter paper substrates

10 µm

10 µm

10 µm

10 µm

(a)

(b)

Figure 11. Photographs of (a) glass slide, 146 (b) SiO2 on Si wafer, 147 and (c) ceramic crucible148

(a)

(b)

(c)


3.3 SYNTHESIS APPROACHES AND TOOLS

3.3.1 Deposition of precursor solution: drop casting

One of the most straightforward thin film synthesis techniques, drop-casting

involves the deposition of solution on a substrate usually using a pipette, dropper or

syringe. Surface tension between substrate and the solution plays one of the most

significant roles in the uniformity of the final synthesised products.

3.3.2 Deposition of precursor solution: spin coating

Spin coating is a commonly used technique for uniform thin film fabrication

where the film solution is deposited at the centre of a substrate that is fixed to a

spinning disk with configurable speed and acceleration settings. Upon contact with the

substrate, centrifugal force spreads the solution outward in all directions, coating the

surface uniformly. Continued rotation forces excess solution off the substrate, hence

thickness can be adjusted based on this parameter and also others such as viscosity and

volatility of the solvent(s) used in the solution and the speed setting itself.

A substrate was fixed using carbon tape (smaller than the substrate) on to a glass

slide cut to fit the suction hole on the spin-coater. The speed settings are detailed in

the appropriate chapter.

Figure 13. Photograph of the spin-coater used for deposition of films.


3.3.3 COF synthesis via solvothermal treatment

Solvothermal annealing involves the use of solvent or solvents to facilitate

synthesis of the precursor molecules through their interactions at relatively high

temperatures. Precise control of the final product can be achieved through

solvothermal synthesis due to the wide range of parameters that can be of influence:

temperature, temperature gradient, pressure, humidity, solvents, time, substrate,

substrate size, solution, solution deposition method, etc. The solvothermal setup for

our work is shown in Figure 14. The petri dish sufficiently seals the beaker and

becomes a closed, condensing environment with heating, eventually leading to heating

of the substrate and solution on the substrate. The temperature of 120 °C was initially

derived from Côté et al.’s synthetic procedure1 but was then adjusted to 115 °C as it

was found (through trial and error) to be the most ‘comfortable’ temperature for our

solvent system, ie, no burn stains or excessively rapid evacuation of solvents.

3.3.4 COF synthesis via solvent-vapour annealing

In solvent vapour annealing (SVA), the sample is exposed to a vapour of a

chosen solvent to induce polymerisation or ordering (curing) using the setup in Figure

15. This key aspect of this method is the gentle and consistent application of the

activating agent (vaporizing solvent) on to the sample. The main driver of

solvothermal annealing, on the other hand, is the high temperature input. This

energetic input is intense and abrupt, forcing chemical reactions to proceed at a fast

Sample substrate

Hotplate

Silica gel with

H₂O

2L beaker

Glass petri dish

Figure 14. Illustration of setup of the solvothermal synthesis


pace; in our experiments, thermal annealing took two hours compared to the 48-72

hours that is required to complete synthesis using solvent vapour annealing. Vapour

annealing is used either on its own or as a complementary technique in various fields

to produce block copolymers149 or materials suitable for photovoltaic

applications150,151. For the fabrication of COFs, vapour annealing has been utilised

twice, both at room temperature to create COF-5 as a thin film,93 and a nanofibrous

COF in bulk form.92 We attempted to emulate the former synthetic procedure and will

be discussed in Chapter 6.

3.3.5 COF synthesis via sonication

Sonochemical synthesis, or sonication, is the use of high intensity ultrasound to

form acoustic cavitation in liquids, where the energy produced is then used to initiate

chemical processes and reactions. The collapse of these cavitation bubbles leads to

local heating, intense pressures, drastic temperature gradients, and jet streams.152

These dramatic effects must be considered along with the increasing overall

temperature of the water bath, and hence the system, as the procedure progresses. This

technique has become particularly utilised in pharmaceutical research for its

effectiveness in particle emulsification, activation and deagglomeration.153 The

synthetic procedure involves the submersion of substrate in a sealed container (4 mL

vial) of the precursor solution for a given time. We found 2 hours to be the right

duration to observe growth on the substrate.

2L beaker

Parafilm

20mL beaker of anisole

Figure 15. Illustration of the setup of the room temperature solvent-vapour assisted synthesis

Silica gel

without H₂O

Sample substrate


3.3.6 Thermal imprinting

Thermal imprinting utilises mechanical deformation to aid in the thermal curing

of the solution drop-casted on the substrate. While this nanolithography derived

method usually involves the use of a template,154,155 this experiment only intend to take

advantage of the effects of pressure and compression on thin film growth. We start

with a spin-coated sample which is then immediately placed in the imprinter with a

clean substrate placed directly on top of it, face down as shown in Figure 16.

3.4 CHARACTERISATION OF COFS

3.4.1 Microscopy

High-resolution images of the COF membranes were obtained with both the

Zeiss Orion NanoFab Helium Ion Microscope (HIM) and Zeiss Sigma Scanning

Electron Microscope (SEM). For sample preparation, the membranes were adhered to

a carbon tape on an SEM specimen stub. An electron flood gun was utilised to

minimize sample charging. Optical images were captured using the Nikon SMZ1500

stereomicroscope and polarised images with the Leica DM6000. Tapping mode atomic

force microscopy (AFM) on a Park NX10 AFM was employed to image the surface

and profile the roughness of membranes obtained through both synthesis methods.

(b)

(a)

Figure 16. Schematic illustration of the thermal imprinter (a), and photograph

of the imprinter (b).


3.4.2 Spectroscopy

Fourier transform infrared (FT-IR, or IR) spectra were collected using the

Nicolet iS50 FTIR spectrometer with 64 scans at 4 cm-1 resolution. No difference in

quality was seen at 128 scans. Analysis on each sample was performed at least twice

to ensure consistency. Data were collected with respect to transmittance %. For

thermally processed samples, the sample was cooled down to room temperature in a

desiccator prior to analysis. Only films that were easily removed that had already

delaminated from the substrate were analysed without the substrate as forced removal

can cause the substrate to chip and contaminate the sample.

3.4.3 Surface area analyses

Surface area analyses were performed on the Micromeritics 3 Flex Physisorption

analyser using argon and at a pressure range between 0.01-0.1 p/p. The free space

values were determined at the end of the analysis instead of beginning to prevent

helium entrapment in the samples.

3.4.4 X-ray diffraction

2Θ and 1° fixed incident angles were used to acquire XRD measurements with

a Rigaku SmartLab diffractometer using a Cu Kα source. Data were fitted using the

Le Bail routine.

3.4.5 Nano-mechanical characterisation

Mechanical analysis of the membranes were performed with a Bruker Hysitron

TI 950 TriboIndenter with a cube corner tip and 1D transducer. A 2000 μN load

function was used, as determined through preliminary load testing of the sample.

Calculated modulus and hardness take into account substrate effects, probe shape and

tip and optical calibrations.

3.4.6 Transmission electron microscopy

Transmission electron microscopy (TEM) is a technique that utilises an electron

beam transmitted through the sample to form an image at high atomic resolutions.


3.5 PARAMETERS EXPLORED THROUGHOUT WORK

A synopsis of the variables explored is provided in Appendix E.

Chapter 4: Results 29

Chapter 4: Results

A series of pilot studies were performed to establish the fundamental parameters

necessary for transferring synthesis procedures originally designed for bulk and single-

layer COF-1s into thin film forms. The success of this relegation depends critically on

solvent wettability, solution evaporation rates, substrate choice, and the technique itself.

4.1 SOLVOTHERMAL FILMS

To determine the parameters most relevant in controlling COF film synthesis, we

started by considering the synthesis technique proposed by Côté et al. in 2005.1 In this

case, the self-condensation or molecular dehydration reaction of the 1,4-benzenediboronic

acid (BDBA) building block leads to the formation of bulk COF-1, a long-range-ordered

2D framework and the pioneering COF. The reaction involves the convergence of three

boronic acid molecules to form a boroxine ring and eliminate three water molecules. Côté

et al.’s solvent choice was a 1:1 mixture of mesitylene and dioxane.1 However, since we

were synthesising a film, we also considered the method conceived by Cui et al. in 2015.5

Heptanoic acid was used as the dissolving solvent for HOPG-confined synthesis of COF-

1.

For the pilot study, we started with a solution of 1 mg of the 1,4 benzenediboronic

acid (BDBA) monomer and 1.5 mL of heptanoic acid in a 4 mL vial. The solution was

then placed in an ultrasonic bath for 20 minutes to completely dissolve the BDBA

powders. In our first experiment, we drop-casted the solution onto a HOPG substrate and

proceeded with the classic solvothermal synthesis in a humid environment (see Section

3.3.3). The SEM image in Figure 17(a) shows the sample post-synthesis. Figure 17(b) and

(c) shows the result on other substrates: an Si wafer (b) and on a graphene on Cu foil

substrate (c). Consistent on all three substrates is the presence of crystallites relatively

evenly distributed across the surface. The HOPG and graphene on Cu foil also illustrate

the effect of an uneven or protruding surface on crystal seeding and nucleation. Crystal

30 Chapter 4: Results

seeding is based on the notion that recrystallisation starts when two random soluble

molecules suspended in a solute interact to create intermolecular forces that leads to the

formation of a basic crystal lattice. This lattice collides with another random molecule and

is extended, and so on. By placing a larger, formed crystal, or a seed, and in the case of

the HOPG and graphene on Cu foil substrates, the elevated textures, the reliance and time

required for random molecule interaction is eliminated. Based on Cui et al.’s results,5 we

assumeed that any evidence of COF monolayer formation is inappreciable at this

magnification and that the crystallites were either residual/unreacted BDBA or COF that

settled as isolated aggregates. It was determined through the infrared spectroscopy shown

in Figure 17(d) that these crystals were the former: BDBA crystallites precipitated out of

the heptanoic acid solution and were left unreacted on the surface of the substrate.

Figure 17. Crystallite distribution of drop-cast solovothermal samples imaged usng SEM; (a) HOPG,

(b) SiO2/Si wafer, and (c) graphene on Cu foil.

(a)

(b)

(c)

100 µm

100 µm

200 µm


As BDBA is only slightly soluble in heptanoic acid, experiments with a variety of

solvents were conducted to understand their influences on the final product. Côté, et al.1

stated that the ideal solution is one in which the BDBA precursor is moderately soluble.

As BDBA is (highly) soluble in ethanol, we attempted to dissolve the BDBA in a 1:1

solution of heptanoic acid and ethanol. The mixture proved successful and dissolved the

BDBA without sonication. The solvothermal synthesis performed however, was not

successful. Based on the IR spectrum in Figure 18, no development of the necessary

boroxine and boronate ester ring bands for COF-1 is observed. Excess peaks in the 2900

cm⁻¹ and 1800 cm⁻¹ region belong to residual heptanoic acid. Figure 19(a) and Figure

20(a) show the final morphology of a BDBA crystals produced from heptanoic acid, and

in Figure 19(b) and Figure 20(b) from the binary solvent solution.

Figure 18. IR spectrum of solvothermally annealed crystallites and precursor powder.


To examine the crystal in greater detail, and in particular to see if the crystal is a

single component or if it contains aggregates, we analysed the heptanoic/ethanol BDBA

crystal using helium ion microscopy. The analysis was unsuccessful on the heptanoic acid

only sample due to the sticky nature of the solvent, which in addition to the already highly-

insulating nature of the crystal made analysis in vacuum difficult without further heating

the sample to remove the solvent and possibly changing the crystal morphology

altogether. The heptanoic acid/ethanol solution was more cooperative, possibly due to less

(a)

(b)

100 µm

100 µm

Figure 19. COF-1 crystallites imaged using a polarizing light microscope; (a) sample prepared

using heptanoic acid solution; (b) sample prepared using heptanoic acid-ethanol solution.

(a)

(b)

40 µm

40 µm

Figure 20. COF-1 crystallites imaged using SEM; (a) sample prepared using heptanoic acid

solution; (b) sample prepared using heptanoic acid-ethanol solution.


heptanoic acid in the system or the esterification creating a solvent with a higher vapour

pressure.

As shown in Figure 21, tens of layers of crystallites build on each other to form a

single, rigidly crafted heptanoic acid/ethanol-produced BDBA aggregate. We found that,

in general, the morphology of the crystallites/aggregates formed was extremely sensitive

to the solvents used. Numerous studies have demonstrated that solvents have a significant

impact on the strength of the intermolecular forces on the faces of the solute crystal. In

other words, the solvent-crystal surface combination determines the bonds affected and

subsequently the final crystal shape and aspect ratio.156,157 Figure 22(a) illustrates the

crystal morphology of an ethanol only solution. Furthering the experiment on solvent

effects, we dissolved BDBA in 0.5:1 heptanoic acid/ethanol, 1:1 ethanol and acetone, and

acetone, and the resulting morphologies are presented in Figure 22(b) and (c) respectively.

At the low BDBA concentration of 1 mg per 1.5 mL, solvothermal synthesis using

heptanoic acid is ineffective in converting BDBA into COF-1. Increasing the

concentration was not possible as BDBA is mostly insoluble in heptanoic acid and a

moderately soluble solvent is instead required for the condensation reaction to lead to the

production of COF-1. While the heptanoic acid/ethanol solution was more soluble,

allowing the dissolving of up to 60 mg of BDBA per 1.5 mL of solvent, we observed a

similar IR spectrum as to that of the heptanoic only solution in Figure 18.


(a)

(a)

(b)

(b)

Figure 21. Heptanoic acid/ethanol solvothermal BDBA crystals high-resolution imaged using helium ion

microscopy; (a) layered crystal morphology of the heptanoic acid/ethanol treated BDBA crystal; (b) close-

up of the crystal layers.

1 µm

200 nm

(a)

(b)

(c)

(d)

Figure 22. Crystal morphologies of BDBA crystals solvothermally synthesised in various solvents imaged

using SEM; (a) ethanol, (b) 0.5:1 heptanoic acid/ethanol, (c) 1:1 ethanol/acetone, (d) acetone .

100 µm

2 µm

2 µm

1 µm

(b)

Fig

ure

18.

Hep

tano

ic

acid

/eth

anol

solv

othe

rma

l

BD

BA

crys

tals

high

-

reso

luti

on

ima

ged

usin

g

heli

um

ion

mic

rosc

opy;

(a)

laye

red

crys

tal

mor

pho

log

y of

the

hept

anoi

c

acid

/eth

anol

trea

ted

BD

BA

crys

tal;

(b)

clos

e-

up

of

the

crys

tal

laye

rs.(

b)

(a)

(a)


4.2 SONICATION

The second technique we explored was inspired by the fabrication of COF-1 and

COF-5 bulk powder via sonochemical synthesis by Yang et al. in 2012.158 Sonication

involves the transformation of the ultrasonic waves into mechanical energy by

inducing pressure variations large enough to cause bubble cavitation collapse in

liquids.159 This technique has become particularly utilised in pharmaceutical research

for its effectiveness in particle emulsification, activation and deagglomeration.153

Based on the work of Yang et al.158, we expected solovochemical treatment to act as a

one-step synthesis, directly forming COF-1 on the surface of samples.

To synthesise a substrate-confined COF-1 film via ultrasonication, the substrate

must be resistant to destruction by (1) ultrasonic waves, and (2) immersion in

heptanoic acid for more than two hours. We tested on the HOPG, SiO2/Si wafer and

graphene on Cu foil and found the foil to be most resistant to disintegration. The HOPG

was shredded almost instantly by the ultrasound, due to the intense mechanical energy

produced breaking the layers of graphite sheets, causing them to exfoliate. Crystals did

not grow at all on the Si wafer. Figure 23(a) presents the graphene on Cu foil sample

after two hours of sonication. We can see that the crystals covered the film with a

uniformity similar to solvothermal synthesis. The agglomeration of crystals at the

bottom and side of the substrate might be due to contact with the glass vial, which

could induce nucleation more vigorously than in regions only in contact with the

solvent. The crystals have a turquoise tint, which we hypothesize to be due to the

leeching of copper +2 ions from the foil due to interaction with the heptanoic acid.

Figure 23(b) shows the result of the second experiment using the same vial of solution,

which was found to have a turquoise tint at the end of the experiment. Compared to

the first experiment, the crystals were noticeably larger, thicker, more vibrant and more

abundant. Note that the resulting shape of the substrate is due to it tearing when being

removed from the vial, not during synthesis.


We were able to retrieve a small sliver of HOPG from one of the experiments

and observe the resulting product using SEM. From observations made on the small

piece alone, it appears that crystal growth was successful and proceeded in a similar

fashion to the foil. Like the foil, crystal agglomeration is observed along the edges and

curved, exposed regions (Figure 24). Broken, damaged crystals were observed (Figure

25(b)) lying randomly on many areas of the films, along with small crystals collected

into depressions, and agglomerations in Figure 25(c). The broken crystals were

assumed to be once longer like those in Figure 23(d). These outcomes were

consistently observed in subsequent experiments, leading us to the conclusion that the

technique would not produce uniform, replicable COF-1 films on the chosen substrate

and solvent mixtures.

Figure 23. Comparing two instances of sonication of graphene on Cu foil in a vial of solution,

with the second experiment using the same vial and solution as well; (a) film morphology of

first experiment; (b) close-up of sparse crystal distribution; (c) film morphology of second

experiment; (d) close-up of crystal distribution.

(a)

(b)

(c)

(d)

3 mm3

20 µm

3 mm

20 µm

3 mm


(b)

(a)

Figure 24. Film on various substrates synthesised via ultrasonication imaged using SEM; (a) graphene on

Cu foil and (b) HOPG.

30 µm

30 µm

Figure 25. Other crystal and particle morphologies observed on a post-sonicated graphene on Cu foil

imaged through SEM; (a) blocks of crystals dispersed randomly on film; (b) crystals appearing to be

broken chunks of a much longer piece; (c) small crystals dispersed randomly on more uneven regions

of the substrates.

(b)

(a)

(c)

20 µm

40 µm

20 µm


4.3 SPIN-COATING

Spin-coating is a commonly used multi-industry technique to fabricate thin films

and film uniformity is one of its main selling points.160,161 Since uniformity of films

was clearly an issue with both drop-casting and sonication, spin-coating was an

appealing deposition technique to explore. The spin-coating experiments began with

finding the optimal speed and acceleration settings for our particular film, taking into

account solvent viscosity and volatility, precursor concentration, substrate dimension,

and solution wetting behaviour on the selected substrate. Unfortunately, this is a highly

specialised, case-by-case technique and required much trial and error to find the right

recipe. Our aim was to spin-coat a uniform wet film for subsequent solvothermal

synthesis on the hot plate.

4.3.1 Spin-coating with heptanoic acid solution

The solution of 1 mg of the 1,4 benzenediboronic acid (BDBA) monomer and

1.5 mL of heptanoic acid was added to an empty 4 mL vial. It was then placed in an

ultrasonic bath for 20 minutes to completely dissolve the BDBA powders. We

identified the ideal settings to consist of a slow initial spin for 10 seconds to cast the

solution on the substrate. If the substrate was stationary, acceleration will either break

the droplet abruptly or push the solution off the substrate completely. The next segment

is the main spin, which spreads the solution evenly and evapourates the solvent. Our

solution required a fast spin in the range of 5000 rpm. Determining this speed plays

into the balance between solvent viscosity and volatility; too fast and most if not all

the solution would have been spun off, too slow and the centrifugal force has little to

no effect on spreading the solution. Finally, we slowed the spin coater to

approximately 1000 rpm, to slow down drying to allow for longer reaction times. It is

important to note that slower speeds also increase thickness of the film, as this

thickness is inversely proportional to the spin speed squared.162 However, the concern

most pressing at this stage is uniformity and as shown in Figure 26, spin coating does

not lead to a uniform distribution of crystallites on the surface. As with the other

explored techniques, only sparse crystal coverage results.


4.3.2 Spin-coating with heptanoic acid/ethanol solution

Since solvents were found to have an effect on BDBA crystal growth and

morphology using other film preparation techniques, the experiment was performed

again using the heptanoic acid/ethanol mixture, with the spin speed adjusted for the

change in solvent properties. This was also not successful in producing a uniform film.

Before proceeding with experimenting with other solvents, the concentration of

precursor molecule was re-evaluated based on the observed density of the crystallites

on the films. While the BDBA was almost insoluble in heptanoic acid, the heptanoic

acid/ethanol mixture easily dissolved 60 mg of BDBA per 1.5mL of solvent.

Compared to 1 mg in the original solution, we began with increasing the concentration

to 10 mg per 1.5 mL solvent only. This change in solution proved to be successful in

achieving a continuous film. Figure 27(a) shows a polarized image of the film

morphology on a Si wafer, and (b) a close-up of the crystal morphology. The same

result was not achieved on the HOPG as seen in Figure 27(c) and (d), similar to the

results obtained for the heptanoic only solution.

While the continuous film was achieved, solvothermal synthesis did not help

with initiating the condensation reactions as shown in the IR spectrum in Figure 28,

which shows the traces of both precursor and synthesised film overlapping almost

perfectly. This suggests that a dry film cannot be converted to COF through a standard

solvothermal synthesis.

(b)

(a)

Figure 26. BDBA solution spin-coated on two different substrates imaged using (a) HIM on the

Si wafer and (b) SEM on the HOPG.

400 nm

100 µm


Figure 28. IR Spectra of thermally annealed spin-coated COF-1 film vs BDBA precursor powder

Figure 29: IR Spectra of thermally annealed spin-coated COF-1 film vs BDBA precursor powder

Figure 27. Comparison of films spin coated and then annealed on two different substrates. (a-b) film on Si

wafer imaged using (a) optical microscopy and (b) helium ion microscopy. (c-d) film on HOPG imaged

using (c) optical microscopy and (d) scanning electron microscopy.

(b)

(a)

(d)

(c)

10 µm

500 µm

400 nm

500 µm


4.4 THERMAL IMPRINTING

We used thermal imprinting to add directional pressure (compressive strain) to

align and densify films solution drop-casted onto a substrate. While this

nanolithography-derived method usually involves the use of a template,154,155 this

experiment only intended to take advantage of the effect of compression on thin film

growth.

20 mg of BDBA in 4 mL of heptanoic acid and ethanol mix (1:1) solution was

drop-casted on a 10×10mm Si wafer. We then spin-coated the solution. A clean

substrate was then placed directly on top of it, face down as shown in Figure 5(a). The

two substrates were secured with a Teflon tape before placing it in the imprinter. A

torque wrench was then used to secure the screws at a pre-determined torque. Finally,

the complete fitting was heated for 3 hours at 120°C on a hot plate.

Four samples were fabricated using the same solution but on different days. Shown in

the SEM images in Figure 29, there were visible differences in morphology, density

and distribution between the four experiments despite using the same precursor

molecule, solution and pressure. This can imply influence from the age of the solution

but when the experiment was repeated with a new solution and on the same day, the

same results as the previous solution were obtained. We see that crystals appear to be

preferentially forming sheets approximately perpendicular to the substrate, implying

the tendency for directionality in thermally imprinted films.

Thermal imprinting has the effect of often producing two substrate-bound films

from a single spin-coated sample. To fasten the screw top to the imprinter vessel, a

torque wrench set to a specific torque was used. While this allows for consistent

loading on all parts of the imprinter, the loading between the clean, face-down

substrate and the spin-coated substrate can vary slightly with each experiment. There

were two possible scenarios here: the clean, face-down substrate can simply press

down on the spin-coated substrate and decouple smoothly after treatment, or, it can

smear the film and the film can become adhered to it. Figure 30 shows post-synthesis

optical images of (a) a spin-coated sample and (b) the corresponding clean substrate,

showing that transfer between the spin-coated substrate and the clean one produces

similar films on both.


Some film inhomogeneity may arise from the spin-coating process. While the

films appear optically uniform, the degree of this uniformity at the nanoscale is unclear

unless analysed under high-magnification. This is not feasible as the spin-coated

sample must be placed in the imprinter immediately to prevent further drying and

morphological change of the crystals.

Thermal imprinting leads to COF-1 formation. The IR spectrum (Figure 31) of

the film reveals the formation of the double boronate ester and boroxine anhydride

bonds associated with COF-1, along with some peaks representing the precursor

molecule still present. Figure 32 shows a variety of other morphologies observed from

repeated experiments. These crystals produced the same spectra as the samples before,

suggesting that they were COF-1, but the macroscale morphologies differ from those

shown in Figure 29, suggesting that more work needed to be done to achieve consistent

outcomes.

(a)

(d)

(b)

(c)

Figure 29. SEM images of morphological variations of thermally imprinted COF-1 films on Si

substrates synthesised on four different days using the same solution; (a) layering of crystals in various

distractions; (b) crystal mass appearing to be more fused together than in (a) due to greater impact

from compression; (c) dense masses of crystals; (d) densest packing of crystals seen in all samples.

1 µm

1 µm

1 µm

1 µm


Figure 31. IR spectra of thermally imprinted COF-1 film and BDBA precursor powder.

(a)

(b)

Figure 30. Thermally imprinted COF-1 film imaged using a stereomicroscope; (a) spin-coated substrate, and

(b) clean substrate.

2 mm

2 mm

B-O

B₃O₃


4.5 A NOTE ON ANALYTICAL TECHNIQUES

In these pilot studies we were not able to take advantage of several analytical

tools such as TEM and XRD, due to the difficulty in obtaining reliable data through

these techniques.

Thin film XRD requires a film that is uniform in the area covered by the beam.

This is ideally about 10 mm in diameter. Since the substrates used were only 5 mm ×

5 mm, the next best solution would be to synthesise multiple samples and scrape off

the product to be placed in a capillary tube. Scraping off the product proved almost

impossible due to the fragility of the HOPG, Si wafers and graphene on Cu foil.

Extending the synthesis technique to a 10 mm × 10 mm substrate was also challenging,

and we made this attempt only on a Si wafer. The solution did not follow the same

wetting behaviour on a larger surface; the droplet simply retreated to its ‘comfortable’

size. The edges of the substrate, which on a 5 mm × 5 mm substrate were useful to

maintain the surface tension necessary to hold the droplet, had no impact on the 10

mm × 10 mm substrate.

Figure 32. Other morphologies observed on thermally imprinted COF-1 films imaged using SEM;

(a) leaf-like crystal growth; (b) uneven distribution of pellet-shaped crystals; (c) sparse masses of

crystal aggregations.

(a)

(b)

(c)

30 µm

1 µm

3 µm


TEM requires thickness in the nanoscale and although the crystals measured

within the range, we identified through helium ion microscopy that each crystal

aggregate is actually composed of tens of layers of smaller crystallite sheets (Figure

21(b)). Under the TEM, this showed up as an opaque blob. In addition, the insulating

nature of the crystals resulted in charging effects that made the already challenging

task of finding a single crystal within the aggregate even harder.

4.6 CONCLUSIONS

The results so far provided evidence that in addition to synthesis method,

solvent, precursor concentration and substrate choice all have considerable effects on

the final morphology of the synthesis product.

The pilot experiments also revealed other variables that may be of the same

significance but that were not explicitly investigated:

• Temperature: it was unclear as to how accurate the temperature was on the

hotplate and if that inaccuracy could have an impact on the product.

• Time: the heptanoic acid sample was still ‘wet’ at the end of the two hours of

solvothermal treatment (i.e., Solvent fumes lingered/was still potent and the film

exhibited quarter-wavelength features consistent with solvent retention), while the

ethanol sample dried within two minutes.

• Water vapour: when does the water vapour stop having an effect on crystal

growth and morphology?

• Apparatus: Côté et al.1 utilised a sealed pyrex vial. A covered petri dish with

silica gel was used instead for our solvothermal experiment.

• Pressure: the sealed Pyrex vial would have accumulated measurable pressure

in comparison to the unsealed petri dish setup. As temperature rises in the sealed

vessel, increased kinetic energy of the liquid turned to vapour molecules results also

in an increase in the vapour pressure.

• Humidity: Ambient humidity was not controlled for in all experiments.

• Deposition consistency: this is more likely to affect distribution than sheet

morphology but must be considered.


In these pilot studies, the technique and solution combinations selected did not

result in the formation of a continuous film in which the precursor molecule was

predominantly converted into COF-1. The heptanoic acid/ethanol solution was

effective for spin-coating but COF-1 formation was not achieved with subsequent

annealing. The heptanoic acid/ethanol solution was also effective for thermal

imprinting, with COF-1 formation obtained. However, a continuous film did not form,

suggesting that the concentration and/or solvent mixture still needed refining. Later

experiments used different mixtures with better results.


Chapter 5: Substrate-Supported

Membranes through Thermal

Processing

5.1 SOLVOTHERMAL

The heptanoic acid/ethanol solvent system used in pilot studies was not able to

dissolve more than 60 mg of precursor per 1.5 mL of solvent. Following BDBA

solubility experiments (see Section 3.1.2), we created a new solvent mixture that

dissolved up to 215 mg of BDBA per 1.5 mL of solvent. Despite this capacity, we

began with the lowest viable concentration and found 55 mg to be sufficient to form a

film. This solution was used for all experiments reported from this point onward in

this chapter unless otherwise stated.

5.1.1 Si wafer and HOPG

Using the new solvent mixture of cyclohexanone, ethanol and ether, which

allows very high concentrations of BDBA to be dissolved, the solvothermal synthesis

technique produced the film synthesised on an Si wafer shown in Figure 33(a) and (b).

Figure 33(c) is a close-up of the underside of the film in (b) imaged using SEM. The

film synthesised on HOPG is shown in Figure 34. The yellow appearance of the film

is caused by the cyclohexanone and is a common oxidation effect after opening and

storage for a long period of time. During synthesis, the cracks seen in the images were

observed to develop as soon as the rigid films formed, detaching the films from the

substrate. The top side of the film synthesised on both the Si wafer and HOPG sheet

appear smooth and lacquer-like macroscopically, with the HOPG having a more

malleable behaviour (coiling) than the Si wafer. Except for the translucent HOPG film,

the opaque films on both substrates possessed a sponge-like texture as seen in Figure

33(b). These structures were even more obvious on the films synthesised on the HOPG

substrate in Figure 34, mainly due to the films being thinner and more translucent.

SEM images revealed that these structures were indentations, as seen in the

depression-like feature at the centre of Figure 33(c). The HOPG films were flexible

and therefore appear to be more susceptible to folding.

50 Chapter 5: Substrate-Supported Membranes through Thermal Processing

(a)

(b)

(c)

Figure 33. COF-1 film on a Si wafer synthesised via thermal annealing imaged using the

stereomicroscope for (a) and (b) and SEM for (c); (a) film morphology post-synthesis; (b) sponge-like

underside of film; (c) close-up of the underside of film.

1.5 mm

2 mm

3 µm

(a)

(b)

Figure 34. COF-1 film on an HOPG substrate synthesised via thermal annealing imaged using a

stereomicroscope; (a) curled-films post-synthesis; (b) close-up of films with spherical droplets

visible.

1.5 mm

1 mm


A repeat experiment after distillation and filtration of the cyclohexanone

produced the results shown in the optical images in Figure 35. Apart from the centre,

the film no longer possessed the yellow tint. The underside of the film also appeared

to be free of the spherical masses. IR spectra (Figure 36) show formation of the

required boron ring group bands in COF-1, corresponding to the boroxine and boronate

ester rings.

Figure 36. IR spectrum of COF-1 film obtained by thermal

annealing with BDBA powder for comparison.

(a)

(b)

Figure 35. COF-1 film thermally annealed with distilled cyclohexanone imaged using a

stereomicroscope; (a) film morphology post-synthesis; (b) sponge-like morphology of the

underside of denser, more opaque films.

2 mm

500 µm


5.1.2 Ceramic crucible

The new solution was also successfully tested on a rougher and more porous

substrate, a ceramic crucible. Just like films obtained from the Si wafer and HOPG,

cracks were prevalent throughout the film. Figure 37(a) shows the smoother top side

of the film and (b) the underside of the film. Figure 37(c) presents a close-up of the

underside, which appears to be composed of small individual crystals within the main

porous wafer-like pieces. Figure 38 presents the IR spectra of various parts of the

wafer. These data indicate that the more powdery forms contain more COF-1 than the

surface of the wafer-like pieces.

5.1.3 Teflon filter paper

We also experimented on a Teflon filter paper, which unlike the ceramic

crucible, does not have walls to control solution flow. Crystal growth, despite the

uneven distribution of the film as shown in Figure 39(a) and (b), is uniform with only

Figure 38. IR spectra of the surface, underside and powder form of the COF-1 film synthesised via

thermal annealing on a ceramic crucible with BDBA powder shown for comparison.

Figure 37. COF-1 film synthesised on a ceramic crucible via thermal annealing imaged using SEM; (a)

Film morphology post-synthesis; (b) close-up of crystal growth and fractures.

(a)

(b)

10 µm

2 µm


one sponge-like morphology identified in (c). However, the IR spectrum in Figure 40

suggests no conversion of the BDBA to COF-1 had occurred, with the film and BDBA

powder traces overlapping each almost perfectly.

The uneven distribution of the film is due to the wetting behaviour of the solution

with respect to the Teflon filter paper. Upon drop-casting, the droplet broke apart and

formed smaller droplets across the paper. Because of the porous nature of the substrate,

the solution was absorbed and disappeared faster than on other substrates. This is not

ideal as it meant faster dispersion of solution, and faster evapouration of the solvent.

The ultimate result is premature precipitation of the BDBA, eliminating the condition

required for the formation of COF-1 networks. The uniform crystal morphology

observed is thought to be the result of both the consistent seeding scaffold that is the

filter net and abundance of the precursor molecule. However, despite the porous nature

of the substrate, cracks were still prevalent, appearing to be influenced by the thicker

nets running perpendicular and connecting the thinner fibres within the filter paper.

(a)

(d)

(b)

(c)

Figure 39. Film synthesised on a Teflon filter paper via thermal annealing imaged using SEM. (a)

continuous, smooth region of the film post-synthesis; (b) porous, sponge-like crystal morphology

dominant on film; (c) cracks appearing to be influenced by the filter web texture; (d) close -up of

the cracks.

50 µm

30 µm

8 µm

10 µm


5.1.4 Ceramic filter paper

Solvothermal synthesis on a ceramic filter paper resulted in a predominantly

smooth continuous film with film morphology similar to that of the Teflon filter paper.

Minimal cracking was observed as shown in Figure 41(a) with a close-up of the

fractures illustrated in (b). As on the Teflon paper, the solution did not coat the entire

surface; the droplet tightened upon contact with the ceramic substrate, suggesting that

the substrate was solvophobic. The IR spectrum obtained from this film is almost

identical to that of the Teflon paper (Figure 40), with no bonds indicative of COF-1

formation present.

Figure 40. IR spectrum of the film formed on teflon filter paper with BDBA powder

data for comparison.


5.2 THERMAL IMPRINTING WITH CYCLOHEXANONE SOLUTION

Using the new solution, we made another attempt at thermal imprinting. At this

stage, the spin-coater was unusable, hence another method to apply the film

homogenously was devised. We utilised the vibrations produced from an ultrasonic

bath to induce ‘crystal sorting’ on the substrate. More details can be found in Appendix

A. The result is a wafer-thin, delicate film as seen in Figure 42(a). The spherical

structures were seen again but this time, also observed under the SEM (b), (c), (d).

(a)

(b)

(c)

Figure 41. SEM images of BDBA film synthesised via solvothermal annealing on a ceramic filter

paper. (a) film morphology post-synthesis; (b) close-up of the porous, sponge-like crystal

morphology dominant on the film; (c) partial coverage of film (right) on the filter paper.

5 µm

15 µm

10 µm


Due to the sizes of the spherical structures observed in both the solvothermally

annealed and thermally imprinted films, we hypothesized them to have an effect on

the severity of cracks. Shrinkage from loss of moisture induces channelling of cracks,

with thickness playing a significant role on the extent of these cracks; cracks do not

propagate if the thickness is less than the length of the crack.163 A repeat experiment

left behind a film with a considerable amount of cyclohexanone still remaining (Figure

43). With further testing, it was discovered that BDBA agglomerates into clusters

when added to cyclohexanone, only dispersing and dissolving when ethanol is added

in equal parts to the cyclohexanone. The agglomerates persist even after distillation of

the cyclohexanone. This synthesis did not led to formation of COF-1, as evident in the

(a)

(d)

(b)

(c)

Figure 42. COF-1 film synthesised via thermal imprinting on an SiO₂ wafer imaged using

stereomicroscope in (a) and the SEM in (b), (c), and (d). (a) film morphology post-synthesis; (b)

spherical masses that make up the 'holey' appearance in (a); (c) close-up of connected spherical

masses; (d) interwoven crystal growth in the less 'holey' regions.

1.5 mm

10 µm

10 µm

25 µm


IR spectra in Figure 44. We do not observe formation of boronate ester and boroxine

anhydride peaks.

Figure 43. Repeat experiment of COF-1 film synthesised via thermal imprinting on an Si wafer imaged

using stereomicroscope in (a) and (b) and the SEM in (c), and (d). (a) film morphology post-synthesis;

(b) optical close-up of the droplet-like features; (c) details of the droplets; (d) a variation of the

droplet feature seen on a different location on the same sample.

(a)

(d)

(b)

(c)

1.5 mm

15 µm

100 µm

500 µm


5.3 THERMAL IMPRINTING IN THE PRESENCE OF WATER

We also attempted the thermal imprinting method with water present by placing

the thermal imprinter in a humid, enclosed chamber similar to the setup for

solvothermal synthesis in Figure 3. As the imprinter is sealed vessel, we drilled a small

hole on opposite sides of the vessel to allow for better exposure to the moisture. Three

identical samples were made and the synthesis proceeded for two hours as per previous

experiments. No films were observed to have formed on any of the three samples

(Figure 45(a)). Instead, crystallites formed in isolation, with what appears to be

deterioration/degradation in the forms of holes and webs prevalent in all of them.

Figure 44. IR spectrum of thermal imprinted film using the cyclohexanone/ethanol/ether solution vs

BDBA precursor powder to show absence of peak shifts or conversion.

Figure 45. COF-1 film synthesised via thermal imprinting with the presence of water imaged using SEM. (a)

crystal distribution post-synthesis; (b) close-up of the web-like structure of crystals due to degradation.

(b)

(a)

15 µm

1.5 µm


5.4 SOLVOTHERMAL SYNTHESIS ON ALUMINIUM FOIL

At this point, we refined the solution further by changing the cyclohexanone with

cyclopentanone, another cyclic ketone with one less ring member than cyclohexanone.

BDBA is more soluble in this solvent, which does not lead to formation of the crystal

agglomerates during synthesis.

We tested the new solution (cyclopentanone) on a scalable substrate, aluminium

foil. The foil was cut to many sizes: 5 mm ×5 mm, 20 mm × 20 mm, 100 mm × 100

mm, 300 mm × 300 mm and 500 mm × 500 mm. The drop-casted solution droplet

dispersed upon contact, either overflowing off the foil if there was excess solution or

spreading in a non-uniform manner across the foil on larger substrates. Unlike the Si

wafer and ceramic crucible, there were no edge barriers to contain the solution (the

edges of the Si wafer effectively hold the droplet due to tension and the ceramic

crucible has walls). However, the resulting film on aluminium foil was always

continuous and formed within five minutes. With this discovery, we experimented on

a much larger foil surface: 10 cm ×10 cm and 15 cm × 15 cm. The result was the same;

the solution proved scalable on an aluminium foil. However, the films delaminate and

as can be seen in the HIM images in Figure 46(a); they curl upon removal. In the SEM

images, a consistent 1 micron thick film is observed to have grown uniformly on the

aluminium foil. The crystal morphology is not porous-like like that on the filter papers.

They films appear to comprise an assemblage of individual crystals that form layers to

make one film entity. Furthermore, we clearly see formation of COF-1 bonds in the IR

(Figure 47).

After repeated experiments, we determined a persistent challenge: controlling

coverage and thickness uniformity with scale, since the act of drop-casting the solution

is similar to simply pouring it. To overcome this problem, we utilised the method of

dip-coating: the solution is placed in a dish big enough to fit the aluminium foil and

the foil is then submerged momentarily for coating. This is unfortunately a wasteful

technique as not only is a lot of solution needed to have the foil immersed, the solution

began gelifying due to the faster solvent evaporation rate on a large surface area, in

less than a minute, rendering it unusable after only 2 to 3 applications.


Figure 47. IR spectrum of COF-1 synthesised via solvothermal annealing on aluminium foil with BDBA

powder for reference.

(a)

(d)

(b)

(c)

30 µm

1 µm

100 µm

1 µm

Figure 46. COF-1 film synthesised via solvothermal annealing on aluminium foil with refined solution

imaged using HIM and SEM. (a) film morphology of film delaminated from the aluminium foil (in the

background); (b) close-up of flat region seen in (a); (c) layering of crystals on the delaminated film;

(d) fracture behaviour of films still adhered to aluminium foil.


5.5 CONCLUSIONS

We synthesised micron-scale COF-1 films successfully by using the new solvent

combination comprising cyclohexanone, ethanol and diethyl ether, which incorporated

an increased concentration of the precursor molecules. These films, however, were

prone to cracks, perhaps due to residual stress stemming from differing thermal

expansion coefficients of the substrate and film. Repeated experiments gave similar

results and have led to the conclusion that synthesis of COF-1 film may be more easily

optimized using a different synthesis method. One such method is described in the

following chapter.

Bibliography 63

Chapter 6: Substrate-Supported

Membranes through Solvent-

Vapour Annealing

6.1 SOLVENT-VAPOUR ANNEALING AT ROOM TEMPERATURE

To avoid some of the difficulties associated with creating homogenous, defect-

free films via solvothermal synthesis, an alternative synthesis technique was

investigated. While requiring a much longer synthesis duration, solvent-vapour

annealing requires no heat input (above room temperature), making it an attractive

alternative to solvothermal synthesis. Also, the activating component is the solvent-

vapour, which allows for a gentler and less discriminating application on the film.

6.1.1 COF-5 films

A room temperature solvent-vapour technique was proposed by Medina et al. for

the synthesis of COF-5 films on a glass slide and Si wafer;93 the films produced were

high quality, and up to 10 microns thick. Solvent-vapour annealing involves the use of

a vaporizing agent, usually a solvent, to cure the film, and in this case to polymerise

the molecules of interest. For the COF-5 film synthesis, 72 hours was required to form

the micron-scale film.

As a first step, Medina’s experiment was replicated successfully and exactly as

described. Adapting the procedure prepared by Medina et al.93, a closed environment

for solvent-vapour annealing was created using a 2 L beaker, with 15 g of indicating

silica gel and a beaker of 20 mL of anisole inside. 1.5 mL of precursor solution was

drop-cast into a low form 5 cm wide (30 mL) porcelain crucible and then placed in the

2 L beaker, which was then sealed with Parafilm and left for 48 hours. A film was

achieved on a glass slide (Figure 48), although it was somewhat cracked due to

dewetting issues and contained regions of different colours. The blotchiness resulting

from the cracks appeared to not have a significant effect on the overall uniformity at

high magnification as seen in Figure 48(c); web-like porous structures were observed

on both the brown and clear regions. A more uniform film with edge-to-edge coverage

was obtained on the Si wafer (Figure 49). Optically, the Si wafer-supported film

appeared to have a different tint compared to the film on the glass slide. On closer

64 Bibliography

inspection under the SEM, this difference is explained by the different morphology, as

the Si wafer network is denser and has regularly spaced cavities or craters.

In addition to using IR to identify COF formation through the presence of the

boronate ester rings (B-O) and the expected resonance for C-O in boroxoles, as seen

in Figure 50, differentiating a successful (COF-5) film from an unsuccessful

(crystallized precursor) one is possible with the naked eye, as shown in Figure 48.

Where the COF-5 film failed to form, the resulting product is a layer of ivory and

brown crystals homogenously distributed on the substrate. After repeated experiments,

the causes of failure were determined to be an imbalance in the BDBA to HHTP ratio,

excess moisture, a chamber volume not proportional to the amount of vaporising

solvent used, and the lack of mobility of the solvent vapours; a wide-mouthed solvent

container is most ideal.

(a)

(d)

(b)

(c)

Figure 48. COF-5 film synthesised via room temperature solvent-vapour assisted annealing on a

glass slide imaged using stereomicroscope in (a) and a close-up in (b) and the SEM in (c), and a

close-up in (d).

1 µm

5 µm

2 µm

500 µm

Bibliography 65

(d)

(b)

(c)

(a)

Figure 49. COF-5 film synthesised via room temperature solvent-vapour assisted annealing on a Si

wafer imaged using stereomicroscope in (a) and a close-up in (b) and the SEM in (c), and a close-up

in (d).

1 µm

8 µm

2 mm

500 µm

Figure 50. IR spectrum of COF-5 film synthesised via solvent-vapour annealing and the COF-5

solution.

66 Bibliography

6.1.2 COF-1

After creating the new solvent mixture of cyclohexanone/ethanol/ether, we

attempted the room temperature synthesis for COF-1. The synthesis was unsuccessful

in producing a film and Figure 52 presents the resulting morphology on the Si wafer.

This film was still wet when photographed. Globular agglomerates were very prevalent

in this particular synthesis procedure. The ethanol and ether would have evaporated

within minutes, leaving the BDBA dissolved/suspended in cyclohexanone for the rest

of the duration. As mentioned before, BDBA is mildly soluble in cyclohexanone, with

the observable outcome being clumping. The IR spectrum in Figure 53 indicates no

formation of the crucial boron-based bonds characteristic of COF-1. Having the new

solution of cyclopentanone/ethanol/ether successfully synthesised COF-1 on the

(a)

(d)

(b)

(c)

Figure 51. Crystal morphology of unsuccessful synthesis of COF-5 film via room temperature

solvent-vapour annealing imaged using imaged using stereomicroscope in (a) and (b) and the SEM in

(c), and (d). (a) film morphology post-synthesis; (b) close-up of BDBA and HHTP crystal

combination; (c) close-up of crystal growth on edge of film; (d) general crystal morphology.

100 µm

200 µm

2 mm

500 µm

Bibliography 67

aluminium foil (see Section 5.4), we made another attempt on the synthesis.

Unfortunately, it was as unsuccessful as with the cyclohexanone/ethanol/ether

solution. At this point, it was unclear if the technique simply does not work for the

COF-1 due to an intrinsic barrier (e.g., a kinetic barrier) or if the substrate was the

cause of the problem. To gain clarification, we tested on a ceramic crucible with first

the cyclohexanone solution then the cyclopentanone solution. Both syntheses proved

successful, although results were more unpredictable with the cyclohexanone solution,

possibly due to the spherical clusters affecting film continuity. Not only did we achieve

100% repeatability with the cyclopentanone solution, the film also formed as a

freestanding membrane and will be further discussed in Chapter 7.

(a)

(d)

(b)

(c)

500 µm

2 mm

150 µm

500 µm

Figure 52. Film synthesised via room temperature solvent-vapour assisted annealing on an SiO₂

wafer imaged using stereomicroscope in (a), (b), and (c) and the SEM in (d). (a) film morphology

after synthesis; (b) close-up of shield like film near the edge of substrate; (c) crystal morphology

if 'shield' was absent; (d) close-up of the maze-like structure in (a).

68 Bibliography

6.2 SOLVENT-VAPOUR ANNEALING WITH THERMAL PROCESSING

The unsuccessful synthesis of COF-1 on both the glass slide and Si wafer suggest

that the formation of COF-1 networks on these substrates may require additional

energy inputs, unlike COF-5. To test this theory, we performed the solvent-vapour

annealing with additional thermal input using the same temperature as in the

solvothermal synthesis with the cyclohexanone/ethanol/ether solution. The experiment

was successful at COF-1 conversion as shown in the IR spectrum in Figure 54. The

film formed is similar in morphology to the COF-5 formed through solvent-vapour

annealing, however residual crystals were visible on the surface in Figure 55(a), with

(b) illustrating the film homogeneity captured using a polarized microscope. The

residual crystals were then analysed using the SEM; images are presented in Figure

55(c) and (d). The crystallites appear to have assembled into masses of geometric webs

across the film. This morphology is unique and has not been produced in any of our

other synthesis approaches. The crystallites were also abundant on the film, such that

XRD analysis was finally possible on one of our samples. The XRD spectrum (Figure

56) shows a peak at approximately 2 = 2.5°, suggesting an eclipsed configuration

instead of the staggered packing, which is typical of bulk COF-1. This eclipsed

stacking, however, can be found in COF-1 after high temperature treatment such as

degassing and guest molecule removal.4 The absence of other peaks may imply that

Figure 53. IR spectrum of film synthesised via solvent-vapour annealing at room temperature with

BDBA precursor powder for comparison.

Bibliography 69

our crystal is oriented, or that our crystallites were quite small. A repeat analysis of the

same sample gave similar results.

Figure 54. IR spectrum of the COF-1 film synthesised via thermal solvent-vapour annealing with a

BDBA powder precursor spectrum for comparison.

70 Bibliography

Figure 56. XRD spectrum of COF-1 film synthesised on a Si wafer via thermal solvent-vapour annealing.

Figure 55. Surface distribution of COF-1 film synthesised on Si wafer via thermal solvent-vapour assisted

annealing imaged using an (a) optical microscope, and (b) polarizing microscope; Crystal (c) distribution

and (d) morphology imaged at high-resolution using SEM.

(d)

(b)

(c)

(a)

3 µm

5 µm

500 µm

500 µm

Bibliography 71

6.3 CONCLUSIONS

Room temperature solvent-vapour annealing is an effective but gentle technique

to synthesise micron-scale COF-5 and COF-1 films. While the film is still susceptible

to cracking, the severity is significantly reduced with respect to the COF-1 films

synthesised via solvothermal processes. This film growth behaviour is seen in samples

synthesised with the cyclohexanone/ethanol/ether solution and the

cyclopentanone/ethanol/ether solution. We attribute successful film formation to the

combination of the correct substrate and solvent choices, since films did not form on

the Si wafer but were successfully created on the ceramic crucible. The cyclohexanone

solution was also observed to be unreliable with respect to repeatability, unlike the

cyclopentanone solution. For completion however, the synthesis was repeated on the

Si wafer but with thermal input, despite the film not being a continuous micron-scale

thick film like those achieved in a ceramic crucible at room temperature.

72 Bibliography

Chapter 7: Self-Supporting COF-1

Membranes

Through multiple experiments and refinement of the solution, we successfully

synthesised COF-1 films on a ceramic crucible using two methods: solvothermal

annealing and room temperature solvent-vapour annealing. Furthermore, these films

were freestanding. In this chapter, we provide a thorough description of the properties

of the films obtained through each technique.

7.1 SYNTHESIS DETAILS

We chose the solvothermal synthesis on a hot plate technique for its

straightforwardness and ease of naked-eye observation of the film formation. To

provide a more thorough investigation and characterisation of the membrane (through

technique comparison), we utilised the room temperature solvent-vapour technique as

it has been proven to work for thin film synthesis on a substrate of the COF-5.

Solvothermal Annealing: A closed environment was established on a hotplate

by using an inverted 2 L beaker to enclose both the synthesis vessel, a low-form

porcelain crucible, and a high-form crucible (3 cm wide, 10 mL) containing 3.5 mL of

indicating silica gel containing 5 mL of DDI water. 1.5 mL of precursor solution was

drop-cast into the synthesis vessel and heated for 60 minutes at 120°C.

Room-Temperature Solvent-Vapour Annealing: Adapting the procedure

prepared by Medina et al.93, a closed environment for solvent-vapour annealing was

created using a 2 L beaker, with 15 g of indicating silica gel and a beaker of 20 mL of

anisole inside. 1.5 mL of precursor solution was drop-cast into a low form 5 cm wide

(30 mL) porcelain crucible and then placed in the 2 L beaker, which was then sealed

with Parafilm and left for 48 hours.

Various concentrations of BDBA in solution, from 1 mg to 650 mg were tested

and 165 mg of BDBA in 4.5 mL of a ternary solvent system of dry cyclopentanone,

ethanol and dry diethyl ether was found to be the minimal concentration required for

a membrane to form via both thermal and room temperature solvent-vapour annealing.

A lower concentration resulted in an emulsion-like formation with irregularly

Bibliography 73

distributed clumps of crystallites coated in cyclopentanone. The right concentration

provided molecular packing sufficient for interparticle bonding is necessary for

continuous film formation. The membrane maintains integrity indefinitely until

exposure to H2O, which is known to degrade COF-1 particles through rehydration of

the boroxine rings to boronic acid.

7.2 CHARACTERISATION OF THE FREESTANDING MEMBRANES

7.2.1 Morphological

Although both synthesis procedures produced continuous films, the films were

distinguished by several differences. The thermally annealed membrane is highly

susceptible to cracking, resulting in 3 mm to 5 mm pieces with a thickness of

approximately 120 microns by the end of synthesis. A thin, dense, lacquer-like top

layer forms rapidly in the first 30-40 minutes of synthesis and delaminates shortly after

the formation of the thicker, more porous bottom layer around the 50 minute mark.

Figure 57(a) shows a cross-sectional view of the morphology of the bottom layer of

the solvothermally annealed membrane, with a close-up of the layer shown in (b). A

similar two-layer membrane with thickness of approximately 80 microns was achieved

via vapour annealing. However, the layers of the vapour annealed membrane do not

delaminate (Figure 57(c)), with both layers remaining attached to one another post-

synthesis. Mechanical force (tapping/knocking) is required to disconnect the two

layers. Cracking of the membrane is minimal, giving membranes 10 mm to15 mm in

lateral dimension on average (based on six repeated experiments). Figure 57(d) shows

the surface morphology of the bottom layer of the vapour annealed membrane, with

Figure 55(e) shows the surface of the top layers of solvothermally annealed membrane.

The stereo-optical image (Figure 57(f)) shows that two crystal morphologies

were present in the solvent-vapour annealed sample: a continuous, homogenous

network of small crystallites and larger dendritic crystal agglomerates measuring 800

μm to 1000 μm in diameter. These larger components were relatively evenly

distributed and comprise approximately 60% of the groundmass. In contrast, the

thermally annealed membrane surface is generally smooth and lacks the crystal

aggregations (Figure 57(b)). The AFM images in Figure 58 reveal similar

morphologies at the nanoscale: the solvothermally annealed sample, shown in (a), is

flat and comprises ~50 nm grains, whereas the vapour annealed sample has smaller

74 Bibliography

(~20 nm grains) which were amalgamated into aggregates that impart nanoscale

roughness.

(d)

(b)

(c)

(e)

(f)

(a)

2 mm

200 µm

100 µm

20 µm

10 µm

50 µm

Figure 57. COF-1 self-supporting membranes synthesised via thermal annealing and vapour annealing. (a)

cross-section of bottom layer of both membranes; (b) surface bottom layer of solvothermally annealed

membrane; (c) cross-section of top and bottom layer of vapour annealed membrane; (d) surface of bottom

layer of the vapour annealed membrane; (e) surface of the top bottom layer of vapour annealed; (f) stereo-

optical image of surface of top layer of solvothermally annealed membrane.

Bibliography 75

7.2.2 Crystallographic

XRD of the thermally annealed sample (Figure 59) reveals little to no BDBA

precursor, suggesting that the yield of COF-1 product was quite high. In contrast, for

the solvent-vapour sample, the BDBA peaks have a higher intensity than the COF-1

peaks. The COF-1 contributions to both spectra show peak patterns more akin to an

eclipsed stacking arrangement than the staggered COF-1 pattern observed for bulk

synthesis.1 The considerable broadening of the (100) peaks on both samples can be

attributed to disorder in the membrane stacking and to small COF-1 crystallite sizes:

Scherrer calculations suggest 107 Å crystallites for the solvothermally annealed

sample and 68 Å crystallites for the vapour annealed sample. Due to the small number

of peaks obtained in the XRD pattern, we were unable to obtain the COF to BDBA

ratio in either sample through full-pattern fitting analyses.

Figure 58. AFM phase images and roughness profile of (a) solvothermally annealed COF-1

and (b) vapour annealed COF-1 membranes

Figure 69. AFM phase images and roughness profile of (A) solvothermally annealed COF-1

76 Bibliography

7.2.3 Gas adsorption measurements

The surface areas of both membranes were determined through Ar gas

adsorption after degassing at 200 °C in N2 atmosphere for 24 hours. The measurements

were taken at 87 K from 0 to 1 Po. Isotherms (Figure 60) typical of a microporous

material were obtained for both samples, with overlapping adsorption and desorption

trends. The thermally annealed membrane has a calculated Brunauer-Emmett-Teller

(BET) surface area of 739 ± 11 m²/g, with the micropore component making up 511

m²/g and the mesopore component 228 m²/g. The solvent-vapour annealed membrane

has a surface area of 579 ± 7 m²/g, with micropore and mesopore portions of 254 m²/g

and 325 m²/g respectively. These values were obtained from the Broekhoff-De Boer t-

plot analysis. Density functional theory (DFT) modelled pore widths calculated for the

thermally annealed sample were 4 Å to 9 Å and 12 Å to 54 Å, while the solvent-vapour

annealed sample had calculated widths between 5 Å to 9 Å, 12 to 18 Å and 25 Å to 35

Å, the greater percentage in the latter two domains. In both cases we attribute the

smallest pores to the intrinsic porosity of the COF-1 solid, whereas the larger pores

likely arise from interstitial spaces between crystallites.

7.2.4 IR spectroscopy

The boronate ester B(OH)2 and boroxine anhydride (B3O3) functional bands in

both products (Figure 61) were present as expected in the 1290 cm-1 to 1340 cm-1

Solvothermal

Vapour annealed

Figure 59. XRD spectra of solvothermally annealed (blue) membrane with a sharper peak and less

evidence of unreacted BDBA than the vapour annealed (green) membrane.

Bibliography 77

range and 690 cm-1 region respectively, with the thermally annealed sample spectra

showing little to no difference pre- and post-degassing. The attenuation of

cyclopentanone peaks at the 3500 cm-1 and 1730 cm-1 regions and narrowing of the

B(OH)2 and B3O3 bands post-degassing were evident in the solvent-vapour sample. In

both samples, no significant changes were observed after degassing.

The IR spectra show an amalgamation/commingling of bands in the 1370 cm-1

to 1250 cm-1 range of B-O, C-C and C-B stretches. The B-O bands were not

symmetrical and the C-C band broadens over the C-B band, which is observed in the

Figure 60. Clean Ar gas adsorption/desorption isotherms of solvothermally annealed (blue) and

vapour annealed (green) membranes.

Figure 73. Pre-degas (darker shade) and post-degas (lighter) FT-IR spectra of TA (blue) and vapour

Figure 61. Pre-degas (darker shade) and post-degas (lighter) FT-IR spectra of TA (blue) and vapour-

annealed (green) membranes.

B-O B-C

B₃O₃

78 Bibliography

starting material. This indicates presence of the BDBA in the synthesised membrane.

The overlapping of bands can be observed in both samples but is more apparent and

extended for the vapour annealed sample before degassing.

7.2.5 Nanoindentation

Loading/unloading curves for each sample (Figure 62) were obtained from six

indents close to the center of the membrane. Instability of the probe as it loads the

surface was observed in the thermally annealed membrane, with slip pop-ins

observable from a load function as low as 500 μN. This resulted in large displacement

depths of up to 2800 nm by the end of each cycle. In contrast, a consistent loading

curve with no failure was achieved at all six indents for the solvent-vapour annealed

film, with a maximum indentation depth of 1000 nm. The evaluated modulus and

hardness of the thermally annealed sample were 2.3 GPa and 0.1 GPa, respectively,

and the values for the solvent-vapour annealed sample were 16.4 GPa and 1 GPa,

respectively.

7.3 DISCUSSION

The growth of self-supporting COF membranes comprises a number of

challenges. Solvent choice is much more critical to creating a COF membrane than it

is for producing bulk COF crystallites. For membrane formation, viscosity, substrate

wettability, and solvent volatility were all significant factors.164,165. The ternary solvent

mixtures used here produced uniform wetting of the substrates as liquids, and retained

that wetting during the polymerization/solidification process. The second major

Figure 62. Nanoindentation loading curves for (a) TA COF-1 and (b) vapour-annealed COF-1

membranes with measurement spacing matrix in onset.

Bibliography 79

challenge around COF film formation is the purity of the product. In bulk COF

synthesis, BDBA typically converts to COF-1 with a yield of about 71%,1 and

unreacted material is removed by rinsing. In micron-thick films, the unreacted

material is predominantly trapped inside the film, where it cannot be easily removed.

The value obtained for the SA sample is comparable to the value obtained by Côté et

al. 1 of 711 m²/g for bulk COF-1 powder. The SVA sample has a lower surface area,

but is within 20% of the bulk powder value. We can obtain a clearer understanding of

the difference in the surface area values from the micropore-mesopore ratios in Table

3 below:

Table 3: Micropore-Mesopore Ratio of Surface Areas

Sample Surface Area (m²/g) Micropore (%) Mesopore (%)

Côté et al. 1 711 83 17

SA-COF-1 739 70 30

SVA-COF-1 579 44 56

The bulk powder COF-1 (Côté et al.¹) has a high micropore component, with a

reported yield of 71% conversion. Although we have a slightly higher surface area for

our solvothermally annealed sample, the micropore component is 70%, which is lower

than Cote’s reported value of 87%. This could suggest a lower yield, with fewer COF-

1 micropores present and small crystallites of residual precursor leading to a high

concentration of intergrain mesopores. The low micropore proportion (46%) for the

solvent-vapour annealed sample correlates with the much lower surface area,

suggesting that the porosity dominated by intergrain mesopores, arising from the

packing of small crystallites of both COF-1 and unreacted benzenediboronic acid.

Our FTIR and XRD data support this interpretation by providing evidence of

retained BDBA. Furthermore, along with incomplete conversion from BDBA to COF-

1, guest solvent molecules were also retained within the film structure, particularly in

the vapour annealed films. Cyclopentanone is still present in the film following the

room temperature vapour annealed synthesis (Figure 61), although the degassing

procedure for gas adsorption measurements eliminates it. The XRD pattern clearly

indicates that there is more retained BDBA in the vapour annealed sample than the

80 Bibliography

solvothermally annealed sample. This implies a more efficient conversion of BDBA

to COF-1 through the thermal process and suggests that the increased temperature may

contribute to the thermal evolution of solvent from the polymer structure. However,

the solvothermally annealed membranes exhibit drawbacks in terms of their structure:

the films have two distinct morphological regimes stacked atop one another, and these

two regions of different film density tend to delaminate from one another. Further, the

film is quite prone to cracking, and has not been synthesised into large lateral

fragments comparable to the vapour annealed samples. This effect maybe correlated

with the faster evaporation and release of solvent in the solvothermally annealed

samples as compared to the vapour annealed.

The mechanical testing provides insight into the effect of these different film

morphologies, as well as elucidating the intrinsic mechanical behaviour of the

framework. The mechanical properties of single-crystal COF-1 were expected to be

highly anisotropic, and will depend critically on the orientation of the crystal. The unit

cell is unstable with respect to sheer forces along the plane of the sheets, whereas the

modulus perpendicular to the sheet direction has its maximum value of 143.7 GPa.166

Other lattice orientations exhibit values between these extremes. Microscopically, our

films comprise near randomly-oriented nanoscale crystallites, and should thus exhibit

averaged and isotropic mechanical response when measured at the film level. The

Young’s modulus and hardness of our thermally annealed sample falls in the same

range as conventional polymers whereas the solvent-vapour annealed sample is stiffer

and has a higher hardness value. Our measured results fall inside of the range of

calculated values for different crystal orientations, suggesting that the intrinsic

structure of the COF-1 contributes to their film mechanical properties. For many

materials, the mechanical response of polycrystalline materials is essentially a spatial

average of the single-crystal properties; however we cannot discount the microscopic

structure of the films as a contributor, since grain-level processes can play a role in the

mechanical response of solid materials, particularly when one of the spatial dimensions

is reduced, such as in a thin film.167-171 The macroscopic morphology of the samples

also has a significant effect on the mechanical response, as the less uniform

solvothermally annealed samples shows clear signs of slippage/breaking as compared

to the much more consistent response of the more homogeneous vapour annealed

films.

Bibliography 81

The observed difference in hardness values between the two film types might be

explained by the hardness parameter being less sensitive to substrate effects; hardness

is associated with the narrower plastic deformation region on the stress-strain curve

and the modulus with the elastic region. Despite the (twice) higher displacement depth

for the solvothermally annealed sample, substrate influence is negligible hence is not

observed in the hardness value.

The consistent measurements and higher modulus value obtained for the vapour

annealed sample can be attributed to the relatively homogeneous, continuous film

produced through this technique. Room temperature solvent-vapour annealing

involves two stages: swelling, when the solvent-vapour interacts with the deposited

material, followed by drying of the film through evaporation of the solvent,149 with

both stages being influential on the resulting crystallinity/quality of the final

product,172 and, in our procedure, each stage taking place over tens of hours. This slow

process produces a uniform, dense layer. In contrast, the thermal annealing procedure

proceeds relatively quickly, with rapid volatilization of solvents and growth of

crystallites. The presence of water vapour and the additional energy injected through

annealing allow for self-healing of the crystallites; the larger crystals and increased

yield of the solvothermally annealed process can be attributed to the presence of H2O

during thermal annealing, as H2O fosters the reversible crystallization conditions for

optimal crystallite production.1

7.4 CONCLUSIONS

We have shown that rigid, self-supporting COF-1 membranes with micron-scale

thickness can be synthesised via two different routes: solvothermal annealing and

room temperature solvent-vapour annealing. Synthesis of the membrane solely by

thermal annealing produces delicate membranes with 3 mm to 5 mm lateral sizes

whereas room temperature solvent-vapour annealing creates larger rigid membranes

10 mm to 15 mm in lateral size, although the solvent-vapour synthesis produces a

product with a significant amount of retained precursor. Both syntheses lead to films

that retain most of the porosity of bulk-synthesised COF-1. Nanoindentation results

confirm that the vapour annealed membranes were stiffer and have a higher indentation

hardness than the solvothermally annealed membranes, with a measured Young’s

modulus of 16.4 GPa. This is the first experimentally-determined value for a COF-1

film and shows that the COF properties were consistent with theoretical predictions

82 Bibliography

and significantly stiffer than conventional polymers. Maximizing the COF-1 synthesis

yield is a key step for obtaining optimal surface area, and further work is required to

determine how increasing the yield will modify the mechanical properties of the

membrane.

Bibliography 83

Chapter 8: Conclusions

This thesis presents an investigation of a range of techniques to synthesise COF-

1 and COF-5 in film form. The pilot studies in chapter 4 brought to light the various

parameters influencing film or crystallite formation. In addition to synthesis technique,

solvent, precursor concentration and choice of substrate were found to be the most

influential parameters to film formation. The solvent or solvent mixture dictates the

final crystal morphology, with precursor concentration affecting the density and

distribution of the crystals and the substrate determining the uniformity of the film

(i.e., the wetting). In addition to establishing these principles, we realized the difficulty

in preparation of these samples for analysis. Challenges included the large thickness

of the films and individual crystallites, and the film uniformity (or lack thereof), which

is crucial for methods such as XRD where a 5 mm × 5 mm uniform film is the

minimum requirement for reliable analysis. These pilot studies effectively narrowed

down the techniques for further exploration and set us on a path for the next

experiments.

Thermally processed routes to COF-1 were examined in Chapter 5. Through

this work, we see again the significance of the solvent, precursor concentration and the

substrate in film formation and the final morphology. A new solution mixture was

required to overcome the nonuniform film formation observed in the pilot studies.

Solubility studies detailed in Chapter 3 assisted in developing a solvent mixture of

cyclohexanone, ethanol and ether that not only provided the right moderately soluble

environment for the precursor but also accommodated a considerably higher

concentration of the precursor than previous solutions. This allowed us the freedom to

investigate the relationship between film formation and concentration and we

determined 55 mg per 1.5 ml of solvent to be the minimum concentration required for

a continuous film to form out of the 215 mg capacity in the ternary solvent system.

This new solution led to thick, optically visible films via solvothermal annealing and

thermal imprinting. However, the films had two issues. First, regardless of technique,

the synthesised films contained clustering of crystallites within the uniform

groundmass. The cyclohexanone was determined to be the cause of this phenomenon

and a similarly structured solvent, cyclopentanone, was found to be a good alternative.

84 Bibliography

This new solvent mixture resolved the issue and led to a cleaner, uniform film. The

second complication was film cracks, perhaps due to contrasting thermal expansion

coefficients between the substrate and film, and/or shrinkage of the film due to the

evolution of water during the synthesis. Experimenting with various substrates did not

have any effect on reducing the severity of the cracks. However, the ceramic crucible

was found to be an excellent platform for synthesizing the films as freestanding forms.

Solvent-vapour annealing methods, which require no thermal input, were

described in Chapter 6. While replication of a literature-reported synthesis was

successful for a COF-5 on a Si wafer and on glass, COF-1 films could not be formed

in the same way. However, success was achieved using a ceramic crucible as substrate,

and again, similar to the solvothermal synthesis, freestanding films were obtained and

with considerable reduction of cracks. A vapour annealing experiment with thermal

input also resulted in successful COF-1 film formation, though not of the same final

morphology (thick films). This suggests that with vapour annealing, the annealing

solvent is as important, if not more so, than substrate choice. However, further

experiments were required to confirm this hypothesis.

In Chapter 7, we describe the self-supporting membranes synthesised through

solvothermal annealing and room temperature solvent-vapour annealing. These

membranes have two layers with distinct morphologies. For the solvothermal sample,

these layers have the tendency to delaminate. Similar to the substrate-confined films,

the freestanding solvothermal membranes were appreciably more prone to cracking

than the vapour annealed membranes. XRD and FTIR data suggest a higher retention

of both solvent and precursor molecules in the vapour annealed membrane.

Mechanical testing of the membranes through nanoindentation revealed the Young's

modulus and hardness of the solvothermal sample to be 2.3 GPa and 0.1 GPa,

respectively. For the vapour annealed sample, the Young's modulus and hardness were

16.4 GPa and 1 GPa, respectively, with the latter being significantly stiffer than

conventional polymers. The successes and failures described in this work emphasize a

number of key parameters, and demonstrate the importance of solvent, substrate

choice, and precursor concentration on the formation of COF-1 films. For example,

the synthesis of COF-1 via room temperature solvent-vapour annealing did not occur

on a Si wafer but was successful on a ceramic crucible and the experiment has been

replicated over 30 times. We also observed the effect of solvent when we changed the

Bibliography 85

cyclohexanone to cyclopentanone due to the tendency for cyclohexanone to

agglomerate BDBA crystals and form globular masses sporadically incorporated into

the film. With cyclopentanone, the film produced was uniform and pristine, detaching

from the ceramic crucible cleanly without leaving behind powders like with its

predecessor, the cyclohexanone. Furthermore, the effects of BDBA precursor

concentration was observed when the formation of a macro scale freestanding COF-1

film was only achievable if BDBA concentration was above 55 mg per 1.5 mL of

solvent. Below that and only individual crystals form.

Table 3 details the experiments performed in this project in chronological order.

Each experiment was repeated at least 15 times for reliability, with those producing

conversion to COF-1 repeated and observed more rigidly.

Table 4: Syntheses in Chronological Order Synthesis

Method

Substrate Solvents and

Ratio

Precursor

per 1.5mL

solvent

Resulting

Chemistry

(through

IR)

Resulting

Morphology

1 Solvothermal Si wafer Heptanoic acid 1 mg BDBA Crystallites

2 Solvothermal HOPG Heptanoic acid 1 mg BDBA Crystallites

3 Solvothermal Si wafer Heptanoic acid /

ethanol (1:1)

1 mg BDBA Crystallites

4 Solvothermal Si wafer Heptanoic

acid/ethanol

(0.5:1)

1 mg

5 Solvothermal Si wafer Ethanol 1 mg BDBA Crystallites

6 Solvothermal Si wafer Ethanol/Acetone

(1:1)


7 Solvothermal Si wafer Acetone 1 mg BDBA Crystallites

8 Sonication Graphene

on Cu

Heptanoic acid 1 mg Unverified Crystallites

9 Sonication HOPG Heptanoic acid 1 mg Unverified Crystallites

10 Spin-coat then

thermal

annealing

Si wafer Heptanoic acid 1 mg BDBA Crystallites

86 Bibliography

11 Spin-coat then

thermal

annealing

HOPG Heptanoic acid 1 mg BDBA Crystallites

12 Spin-coat then

thermal

annealing

Si wafer Heptanoic acid

/ethanol


13 Spin-coat then

thermal

annealing

HOPG Heptanoic acid

/ethanol


14 Spin-coat then

thermal

annealing

Si wafer Heptanoic acid

/ethanol

10 mg BDBA Film

15 Spin-coat then

thermal

annealing

HOPG Heptanoic

acid/ethanol


17 Thermal

imprinting

Si wafer Heptanoic

acid/ethanol

10 mg COF-1 Partial film

19 Solvothermal Si wafer Cyclohexanone/

ethanol/ether

55 mg COF-1 Film

20 Solvothermal HOPG Cyclohexanone/

ethanol/ether

55 mg COF-1 Film

21 Thermal

Imprinting

Si wafer Cyclohexanone/

ethanol/ether

55 mg BDBA Film

22 Thermal

Imprinting w/

H2O


ethanol/ether

55 mg BDBA Film

23 Solvothermal Ceramic

crucible

Cyclohexanone/

ethanol/ether

55 mg COF-1 Film

24 Solvothermal Teflon

filter paper

Cyclohexanone/

ethanol/ether

55 mg BDBA Film


filter paper

Cyclohexanone/

ethanol/ether

55 mg BDBA Film

26 Solvothermal Aluminiu

m foil

Cyclopentanone

/ethanol/ether

55 mg COF-1 Film

Bibliography 87

27 Room

temperature

SVA

Si wafer Acetone/ethanol

per Medina et

al.84

55 mg COF-5 Film

28 Room

temperature

SVA


ethanol/ether

55 mg BDBA Film

29 Room

temperature

SVA

Si wafer Cyclopentanone

/ethanol/ether

55 mg BDBA Film

30 Room

temperature

SVA

Ceramic

crucible

Cyclohexanone/

ethanol/ether

55 mg COF-1 Film

31 Thermal SVA Si wafer Cyclohexanone/

ethanol/ether

55mg COF-1 Film


crucible

Cyclopentanone

/ethanol/ether

55 mg COF-1 Freestanding

film

33 Room

temperature

SVA

Ceramic

crucible

Cyclopentanone

/ethanol/ether

55 mg COF-1 Freestanding

film

Based on the work accomplished in this thesis, our future goal is to increase the COF-

1 conversion efficiency (i.e., reduce the proportion of unreacted BDBA), and increase

film size by further refining the solution and other experimental parameters such as

duration, vaporizing solvent and/or substrate. We will also extend this work on 2D-

COFs to 3D type COFs. Synthesising COFs as freestanding membranes makes them

easily integrated in engineered systems such as filtering, sensing and gas storage, and

as such these membranes may prove suitable for a range of applications.

Future Work

This Master’s project will continue to a PhD project focusing on translating/adapting

the methods developed in this project to synthesising other COF types, including

moving from 2D (plane-stacked COFs) to 3D COFs. Candidate systems will include

conducting COFs and COFs in which the reversible bonds can be converted to

irreversible bonds, which will be good candidates for a range of applications where

chemical robustness is required.

88 Bibliography

Bibliography

1 Côté, A. P., Benin, A. I., Ockwig, N. W., Keeffe, M., Matzger, A. J. & Yaghi,

O. M. Porous, Crystalline, Covalent Organic Frameworks. Science 310, 1166

(2005).

2 Plas, J., Ivasenko, O., Martsinovich, N., Lackinger, M. & De Feyter, S.

Nanopatterning of a covalent organic framework host-guest system. Chem.

Commun. 52, 68-71 (2016).

3 Feng, X., Ding, X. S. & Jiang, D. L. Covalent organic frameworks. Chem. Soc.

Rev. 41, 6010-6022 (2012).

4 Waller, P. J., Lyle, S. J., Popp, T. M. O., Diercks, C. S., Reimer, J. A. & Yaghi,

O. M. Chemical Conversion of Linkages in Covalent Organic Frameworks. J.

Am. Chem. Soc. 138, 15519-15522 (2016).

5 Cui, D., MacLeod, J. M., Ebrahimi, M., Perepichka, D. F. & Rosei, F. Solution

and air stable host/guest architectures from a single layer covalent organic

framework. Chem. Commun. 51, 16510-16513 (2015).

6 Smith, B. J., Parent, L. R., Overholts, A. C., Beaucage, P. A., Bisbey, R. P.,

Chavez, A. D., Hwang, N., Park, C., Evans, A. M., Gianneschi, N. C. &

Dichtel, W. R. Colloidal Covalent Organic Frameworks. ACS Cent. Sci. 3, 58-

65 (2017).

7 Li, G., Zhang, K. & Tsuru, T. Two-Dimensional Covalent Organic Framework

(COF) Membranes Fabricated via the Assembly of Exfoliated COF

Nanosheets. ACS Appl. Mater. Interfaces 9, 8433-8436 (2017).

8 Lu, H., Wang, C., Chen, J. J., Ge, R. L., Leng, W. G., Dong, B., Huang, J. &

Gao, Y. N. A novel 3D covalent organic framework membrane grown on a

porous alpha-Al2O3 substrate under solvothermal conditions. Chem. Commun.

51, 15562-15565 (2015).

9 Fan, H., Mundstock, A., Feldhoff, A., Knebel, A., Gu, J., Meng, H. & Caro, J.

Covalent Organic Framework-Covalent Organic Framework Bilayer

Membranes for Highly Selective Gas Separation. J Am Chem Soc 140, 10094-

10098 (2018).

10 Wang, Y., Li, J. P., Yang, Q. Y. & Zhong, C. L. Two-Dimensional Covalent

Triazine Framework Membrane for Helium Separation and Hydrogen

Purification. ACS Appl. Mater. Interfaces 8, 8694-8701 (2016).

11 Berger, R. Ancient Egyptian radiocarbon chronology. Phil. Trans. R. Soc.

Lond. A 269, 23-36 (1970).

12 Lennartson, A. in The Chemical Works of Carl Wilhelm Scheele 11-17

(Springer, 2017).

13 Kiefer, S. & Robens, E. Some intriguing items in the history of volumetric and

gravimetric adsorption measurements. J. Therm. Anal. Calorim. 94, 613-618

(2008).

14 Masters, A. F. & Maschmeyer, T. Zeolites–From curiosity to cornerstone.

Microporous Mesoporous Mat. 142, 423-438 (2011).

15 Chai, S. W., Kothare, M. V. & Sircar, S. Efficiency of nitrogen desorption from

LiX zeolite by rapid oxygen purge in a pancake adsorber. Aiche J. 59, 365-368

(2013).

Bibliography 89

16 Santos, J., Magalhaes, F. & Mendes, A. Contamination of zeolites used in

oxygen production by PSA: effects of water and carbon dioxide. Ind. Eng.

Chem. Res. 47, 6197-6203 (2008).

17 Rangnekar, N., Mittal, N., Elyassi, B., Caro, J. & Tsapatsis, M. Zeolite

membranes–a review and comparison with MOFs. Chem. Soc. Rev. 44, 7128-

7154 (2015).

18 Kitagawa, S., Kitaura, R. & Noro, S. i. Functional porous coordination

polymers. Angewandte Chemie International Edition 43, 2334-2375 (2004).

19 Chen, X., Addicoat, M., Jin, E., Xu, H., Hayashi, T., Xu, F., Huang, N., Irle, S.

& Jiang, D. Designed synthesis of double-stage two-dimensional covalent

organic frameworks. Sci Rep 5, 14650 (2015).

20 Diercks, C. S. & Yaghi, O. M. The atom, the molecule, and the covalent

organic framework. Science 355, eaal1585 (2017).

21 Lim, B. W. & Suh, M. C. Simple fabrication of a three-dimensional porous

polymer film as a diffuser for organic light emitting diodes. Nanoscale 6,

14446-14452 (2014).

22 Tsutsui, T., Yahiro, M., Yokogawa, H., Kawano, K. & Yokoyama, M.

Doubling coupling‐out efficiency in organic light‐emitting devices using a thin

silica aerogel layer. Adv. Mater. 13, 1149-1152 (2001).

23 Wu, S., Wang, G., Xue, Z., Ge, F., Zhang, G., Lu, H. & Qiu, L. Organic field-

effect transistors with macroporous semiconductor films as high-performance

humidity sensors. ACS Appl. Mater. Interfaces 9, 14974-14982 (2017).

24 Wu, G., Huang, J., Zang, Y., He, J. & Xu, G. Porous field-effect transistors

based on a semiconductive metal–organic framework. J. Am. Chem. Soc. 139,

1360-1363 (2016).

25 Azarova, N. A., Owen, J. W., McLellan, C. A., Grimminger, M. A., Chapman,

E. K., Anthony, J. E. & Jurchescu, O. D. Fabrication of organic thin-film

transistors by spray-deposition for low-cost, large-area electronics. Org.

Electron. 11, 1960-1965 (2010).

26 Forrest, S. R. The path to ubiquitous and low-cost organic electronic appliances

on plastic. Nature 428, 911 (2004).

27 Trixler, F., Markert, T., Lackinger, M., Jamitzky, F. & Heckl, W. M.

Supramolecular Self‐Assembly Initiated by Solid–Solid Wetting. Chemistry–

A European Journal 13, 7785-7790 (2007).

28 Díaz, U. & Corma, A. Ordered covalent organic frameworks, COFs and PAFs.

From preparation to application. Coord. Chem. Rev. 311, 85-124 (2016).

29 Haino, T. Molecular-recognition-directed formation of supramolecular

polymers. Polym. J. 45, 363 (2013).

30 Sun, B., Zhu, C.-H., Liu, Y., Wang, C., Wan, L.-J. & Wang, D. Oriented

covalent organic framework film on graphene for robust ambipolar vertical

organic field-effect transistor. Chemistry of Materials 29, 4367-4374 (2017).

31 Olajire, A. A. Recent advances in the synthesis of covalent organic frameworks

for CO2 capture. J. CO2 Util. 17, 137-161 (2017).

32 Kim, S., Lee, W. H., Mun, J., Lee, H. S. & Park, Y. D. Marginal solvents

preferentially improve the molecular order of thin polythiophene films. RSC

Adv. 6, 23640-23644 (2016).

33 Wei, J., Sun, Z., Luo, W., Li, Y., Elzatahry, A. A., Al-Enizi, A. M., Deng, Y.

& Zhao, D. New insight into the synthesis of large-pore ordered mesoporous

materials. J. Am. Chem. Soc. 139, 1706-1713 (2017).

90 Bibliography

34 Shu, Y., Wang, J., Tian, Y., Liang, X., Lin, S. & Ma, B. Thermal Imprint

Introduced Crystallization of A Solution Processed Subphthalocyanine Thin

Film. Adv. Mater. Interfaces 3, 1600179 (2016).

35 Lever, A. The phthalocyanines—molecules of enduring value; a two‐

dimensional analysis of redox potentials. J. Porphyr. Phthalocyanines 3, 488-

499 (1999).

36 Ghazi, Z. A., Zhu, L., Wang, H., Naeem, A., Khattak, A. M., Liang, B., Khan,

N. A., Wei, Z., Li, L. & Tang, Z. Efficient Polysulfide Chemisorption in

Covalent Organic Frameworks for High‐Performance Lithium‐Sulfur

Batteries. Adv. Energy Mater. 6, 1601250 (2016).

37 Elschner, A., Kirchmeyer, S., Lovenich, W., Merker, U. & Reuter, K. PEDOT:

principles and applications of an intrinsically conductive polymer. (CRC

Press, 2010).

38 Yin, X., Wu, F., Fu, N., Han, J., Chen, D., Xu, P., He, M. & Lin, Y. Facile

synthesis of poly (3, 4-ethylenedioxythiophene) film via solid-state

polymerization as high-performance Pt-free counter electrodes for plastic dye-

sensitized solar cells. ACS Appl. Mater. Interfaces 5, 8423-8429 (2013).

39 Colson, J. W., Woll, A. R., Mukherjee, A., Levendorf, M. P., Spitler, E. L.,

Shields, V. B., Spencer, M. G., Park, J. & Dichtel, W. R. Oriented 2D Covalent

Organic Framework Thin Films on Single-Layer Graphene. Science 332, 228-

231 (2011).

40 Lohse, M. S. & Bein, T. Covalent Organic Frameworks: Structures, Synthesis,

and Applications. Adv. Funct. Mater. 28, 71 (2018).

41 Cao, S., Li, B., Zhu, R. & Pang, H. Design and synthesis of covalent organic

frameworks towards energy and environment fields. Chem. Eng. J. 355, 602-

623 (2019).

42 Zhang, S., Yang, Q., Wang, C., Luo, X., Kim, J., Wang, Z. & Yamauchi, Y.

Porous Organic Frameworks: Advanced Materials in Analytical Chemistry.

Adv. Sci., 1801116 (2018).

43 Chen, Q., Dalapati, S. & Jiang, D. in Comprehensive Supramolecular

Chemistry II (ed Jerry L. Atwood) 271-290 (Elsevier, 2017).

44 Ma, X. & Scott, T. F. Approaches and challenges in the synthesis of three-

dimensional covalent-organic frameworks. Communications Chemistry 1

(2018).

45 Uribe-Romo, F. J., Doonan, C. J., Furukawa, H., Oisaki, K. & Yaghi, O. M.

Crystalline Covalent Organic Frameworks with Hydrazone Linkages. J. Am.

Chem. Soc. 133, 11478-11481 (2011).

46 Wu, M., Chen, G., Ma, J., Liu, P. & Jia, Q. Fabrication of cross-linked

hydrazone covalent organic frameworks by click chemistry and application to

solid phase microextraction. Talanta 161, 350-358 (2016).

47 Zhang, K., Cai, S.-L., Yan, Y.-L., He, Z.-H., Lin, H.-M., Huang, X.-L., Zheng,

S.-R., Fan, J. & Zhang, W.-G. Construction of a hydrazone-linked chiral

covalent organic framework–silica composite as the stationary phase for high

performance liquid chromatography. J. Chromatogr. A 1519, 100-109 (2017).

48 Chakravarty, C., Mandal, B. & Sarkar, P. Multifunctionalities of an Azine-

Linked Covalent Organic Framework: From Nanoelectronics to Nitroexplosive

Detection and Conductance Switching. J. Phys. Chem. C 122, 3245-3255

(2018).

Bibliography 91

49 Dalapati, S., Jin, S. B., Gao, J., Xu, Y. H., Nagai, A. & Jiang, D. L. An Azine-

Linked Covalent Organic Framework. J. Am. Chem. Soc. 135, 17310-17313

(2013).

50 Li, Z. P., Zhi, Y. F., Feng, X., Ding, X. S., Zou, Y. C., Liu, X. M. & Mu, Y.

An Azine-Linked Covalent Organic Framework: Synthesis, Characterisation

and Efficient Gas Storage. Chem.-Eur. J. 21, 12079-12084 (2015).

51 Sick, T., Hufnagel, A. G., Kampmann, J., Kondofersky, I., Calik, M., Rotter,

J. M., Evans, A., Doblinger, M., Herbert, S., Peters, K., Bohm, D., Knochel,

P., Medina, D. D., Fattakhova-Rohlfing, D. & Bein, T. Oriented Films of

Conjugated 2D Covalent Organic Frameworks as Photocathodes for Water

Splitting. J. Am. Chem. Soc. 140, 2085-2092 (2018).

52 Wang, Y., Chen, J., Wang, G., Li, Y. & Wen, Z. Perfluorinated Covalent

Triazine Framework Derived Hybrids for the Highly Selective

Electroconversion of Carbon Dioxide into Methane. Angewandte Chemie 130,

13304-13308 (2018).

53 Das, P. & Mandal, S. K. A dual-functionalized, luminescent and highly

crystalline covalent organic framework: molecular decoding strategies for

VOCs and ultrafast TNP sensing. J. Mater. Chem. A 6, 16246-16256 (2018).

54 Halder, A., Karak, S., Addicoat, M., Bera, S., Chakraborty, A., Kunjattu, S. H.,

Pachfule, P., Heine, T. & Banerjee, R. Ultrastable Imine-Based Covalent

Organic Frameworks for Sulfuric Acid Recovery: An Effect of Interlayer

Hydrogen Bonding. Angew. Chem.-Int. Edit. 57, 5797-5802 (2018).

55 He, S. J., Zeng, T., Wang, S. H., Niu, H. Y. & Cai, Y. Q. Facile Synthesis of

Magnetic Covalent Organic Framework with Three-Dimensional Bouquet-

Like Structure for Enhanced Extraction of Organic Targets. ACS Appl. Mater.

Interfaces 9, 2959-2965 (2017).

56 Kandambeth, S., Mallick, A., Lukose, B., Mane, M. V., Heine, T. & Banerjee,

R. Construction of Crystalline 2D Covalent Organic Frameworks with

Remarkable Chemical (Acid/Base) Stability via a Combined Reversible and

Irreversible Route. J. Am. Chem. Soc. 134, 19524-19527 (2012).

57 Karak, S., Kandambeth, S., Biswal, B. P., Sasmal, H. S., Kumar, S., Pachfule,

P. & Banerjee, R. Constructing Ultraporous Covalent Organic Frameworks in

Seconds via an Organic Terracotta Process. J. Am. Chem. Soc. 139, 1856-1862

(2017).

58 Thote, J., Aiyappa, H. B., Kumar, R. R., Kandambeth, S., Biswal, B. P., Shinde,

D. B., Roy, N. C. & Banerjee, R. Constructing covalent organic frameworks in

water via dynamic covalent bonding. IUCrJ 3, 402-407 (2016).

59 Vitaku, E. & Dichtel, W. R. Synthesis of 2D Imine-Linked Covalent Organic

Frameworks through Formal Transimination Reactions. J. Am. Chem. Soc.

139, 12911-12914 (2017).

60 Wang, P., Xu, Q., Li, Z. P., Jiang, W. M., Jiang, Q. H. & Jiang, D. L.

Exceptional Iodine Capture in 2D Covalent Organic Frameworks. Adv. Mater.

30, 7 (2018).

61 Zhang, G., Tsujimoto, M., Packwood, D., Duong, N. T., Nishiyama, Y.,

Kadota, K., Kitagawa, S. & Horike, S. Construction of a Hierarchical

Architecture of Covalent Organic Frameworks via a Postsynthetic Approach.

J. Am. Chem. Soc. 140, 2602-2609 (2018).

62 Zhang, G. Y., Li, X. L., Liao, Q. B., Liu, Y. F., Xi, K., Huang, W. Y. & Jia, X.

D. Water-dispersible PEG-curcumin/amine-functionalized covalent organic

92 Bibliography

framework nanocomposites as smart carriers for in vivo drug delivery. Nat.

Commun. 9, 11 (2018).

63 Colson, J. W., Mann, J. A., DeBlase, C. R. & Dichtel, W. R. Patterned Growth

of Oriented 2D Covalent Organic Framework Thin Films on Single-Layer

Graphene. J. Polym. Sci. Pol. Chem. 53, 378-384 (2015).

64 Ding, X. S., Chen, L., Honsho, Y., Feng, X., Saenpawang, O., Guo, J. D.,

Saeki, A., Seki, S., Irle, S., Nagase, S., Parasuk, V. & Jiang, D. L. An n-

Channel Two-Dimensional Covalent Organic Framework. J. Am. Chem. Soc.

133, 14510-14513 (2011).

65 Ding, X. S., Guo, J., Feng, X. A., Honsho, Y., Guo, J. D., Seki, S., Maitarad,

P., Saeki, A., Nagase, S. & Jiang, D. L. Synthesis of Metallophthalocyanine

Covalent Organic Frameworks That Exhibit High Carrier Mobility and

Photoconductivity. Angew. Chem.-Int. Edit. 50, 1289-1293 (2011).

66 Spitler, E. L., Colson, J. W., Uribe-Romo, F. J., Woll, A. R., Giovino, M. R.,

Saldivar, A. & Dichtel, W. R. Lattice Expansion of Highly Oriented 2D

Phthalocyanine Covalent Organic Framework Films. Angew. Chem.-Int. Edit.

51, 2623-2627 (2012).

67 Spitler, E. L. & Dichtel, W. R. Lewis acid-catalysed formation of two-

dimensional phthalocyanine covalent organic frameworks. Nat. Chem. 2, 672-

677 (2010).

68 Chen, X., Addicoat, M., Jin, E. Q., Zhai, L. P., Xu, H., Huang, N., Guo, Z. Q.,

Liu, L. L., Irle, S. & Jiang, D. L. Locking Covalent Organic Frameworks with

Hydrogen Bonds: General and Remarkable Effects on Crystalline Structure,

Physical Properties, and Photochemical Activity. J. Am. Chem. Soc. 137, 3241-

3247 (2015).

69 Diercks, C., Lin, S., Zhang, Y. B., Chang, C. & Yaghi, O. Covalent organic

frameworks comprising cobalt porphyrins for the electrocatalytic reduction of

CO 2 in water. Abstr. Pap. Am. Chem. Soc. 252, 2 (2016).

70 Liao, H. P., Wang, H. M., Ding, H. M., Meng, X. S., Xu, H., Wang, B. S., Ai,

X. P. & Wang, C. A 2D porous porphyrin-based covalent organic framework

for sulfur storage in lithium sulfur batteries. J. Mater. Chem. A 4, 7416-7421

(2016).

71 Lin, G. Q., Ding, H. M., Chen, R. F., Peng, Z. K., Wang, B. S. & Wang, C. 3D

Porphyrin-Based Covalent Organic Frameworks. J. Am. Chem. Soc. 139, 8705-

8709 (2017).

72 Nagai, A., Chen, X., Feng, X., Ding, X. S., Guo, Z. Q. & Jiang, D. L. A

Squaraine-Linked Mesoporous Covalent Organic Framework. Angew. Chem.-

Int. Edit. 52, 3770-3774 (2013).

73 Wan, S., Gandara, F., Asano, A., Furukawa, H., Saeki, A., Dey, S. K., Liao, L.,

Ambrogio, M. W., Botros, Y. Y., Duan, X. F., Seki, S., Stoddart, J. F. & Yaghi,

O. M. Covalent Organic Frameworks with High Charge Carrier Mobility.

Chemistry of Materials 23, 4094-4097 (2011).

74 Jin, S. B., Sakurai, T., Kowalczyk, T., Dalapati, S., Xu, F., Wei, H., Chen, X.,

Gao, J., Seki, S., Irle, S. & Jiang, D. L. Two-Dimensional Tetrathiafulvalene

Covalent Organic Frameworks: Towards Latticed Conductive Organic Salts.

Chem.-Eur. J. 20, 14608-14613 (2014).

75 Tilford, R. W., Gemmill, W. R., zur Loye, H. C. & Lavigne, J. J. Facile

synthesis of a highly crystalline, covalently linked porous boronate network.

Chemistry of Materials 18, 5296-5301 (2006).

Bibliography 93

76 Côté, A. P., El-Kaderi, H. M., Furukawa, H., Hunt, J. R. & Yaghi, O. M.

Reticular synthesis of microporous and mesoporous 2D covalent organic

frameworks. J. Am. Chem. Soc. 129, 12914-+ (2007).

77 El-Kaderi, H. M., Hunt, J. R., Mendoza-Cortes, J. L., Côté, A. P., Taylor, R.

E., O'Keeffe, M. & Yaghi, O. M. Designed synthesis of 3D covalent organic

frameworks. Science 316, 268-272 (2007).

78 Hunt, J. R., Doonan, C. J., LeVangie, J. D., Côté, A. P. & Yaghi, O. M.

Reticular synthesis of covalent organic borosilicate frameworks. J. Am. Chem.

Soc. 130, 11872-11873 (2008).

79 Mizoshita, N., Ikai, M., Tani, T. & Inagaki, S. Hole-transporting periodic

mesostructured organosilica. J. Am. Chem. Soc. 131, 14225-14227 (2009).

80 Wan, S., Guo, J., Kim, J., Ihee, H. & Jiang, D. L. A Belt-Shaped, Blue

Luminescent, and Semiconducting Covalent Organic Framework. Angew.

Chem.-Int. Edit. 47, 8826-8830 (2008).

81 Dogru, M., Sonnauer, A., Gavryushin, A., Knochel, P. & Bein, T. A Covalent

Organic Framework with 4 nm open pores. Chem. Commun. 47, 1707-1709

(2011).

82 Bunck, D. N. & Dichtel, W. R. Internal Functionalization of Three-

Dimensional Covalent Organic Frameworks. Angew. Chem.-Int. Edit. 51,

1885-1889 (2012).

83 Jiang, Y., Huang, W., Wang, J., Wu, Q., Wang, H., Pan, L. & Liu, X. Green,

scalable and morphology controlled synthesis of nanofibrous covalent organic

frameworks and their nanohybrids through a vapour-assisted solid-state

approach. J. Mater. Chem. A 2, 8201-8204 (2014).

84 Medina, D. D., Rotter, J. M., Hu, Y. H., Dogru, M., Werner, V., Auras, F.,

Markiewicz, J. T., Knochel, P. & Bein, T. Room Temperature Synthesis of

Covalent-Organic Framework Films through Vapour-Assisted Conversion. J.

Am. Chem. Soc. 137, 1016-1019 (2015).

85 Liu, Y. Z., Ma, Y. H., Zhao, Y. B., Sun, X. X., Gandara, F., Furukawa, H., Liu,

Z., Zhu, H. Y., Zhu, C. H., Suenaga, K., Oleynikov, P., Alshammari, A. S.,

Zhang, X., Terasaki, O. & Yaghi, O. M. Weaving of organic threads into a

crystalline covalent organic framework. Science 351, 365-369 (2016).

86 Huang, N., Zhai, L. P., Coupry, D. E., Addicoat, M. A., Okushita, K.,

Nishimura, K., Heine, T. & Jiang, D. L. Multiple-component covalent organic

frameworks. Nat. Commun. 7, 12 (2016).

87 Dalapati, S., Jin, E. Q., Addicoat, M., Heine, T. & Jiang, D. L. Highly Emissive

Covalent Organic Frameworks. J. Am. Chem. Soc. 138, 5797-5800 (2016).

88 Mulzer, C. R., Shen, L. X., Bisbey, R. P., McKone, J. R., Zhang, N., Abruna,

H. D. & Dichtel, W. R. Superior Charge Storage and Power Density of a

Conducting Polymer-Modified Covalent Organic Framework. ACS Central

Sci. 2, 667-673 (2016).

89 Shi, X., Wang, R., Xiao, A., Jia, T., Sun, S.-P. & Wang, Y. Layer-by-Layer

Synthesis of Covalent Organic Frameworks on Porous Substrates for Fast

Molecular Separations. ACS Applied Nano Materials 1, 6320-6326 (2018).

90 Shao, P., Li, J., Chen, F., Ma, L., Li, Q., Zhang, M., Zhou, J., Yin, A., Feng,

X. & Wang, B. Flexible Films of Covalent Organic Frameworks with Ultralow

Dielectric Constants under High Humidity. Angew Chem Int Ed Engl 57,

16501-16505 (2018).

94 Bibliography

91 Zhang, J. Phase transformation in two-dimensional covalent organic

frameworks under compressive loading. Phys Chem Chem Phys 20, 29462-

29471 (2018).

92 Ge, Y., Zhou, H., Ji, Y., Ding, L., Cheng, Y., Wang, R., Yang, S., Liu, Y., Wu,

X. & Li, Y. Understanding Water Adsorption and the Impact on CO2 Capture

in Chemically Stable Covalent Organic Frameworks. The Journal of Physical

Chemistry C 122, 27495-27506 (2018).

93 Zhou, Z., Zhang, X., Xing, L., Liu, J., Kong, A. & Shan, Y. Copper-assisted

thermal conversion of microporous covalent melamine-boroxine frameworks

to hollow B, N-codoped carbon capsules as bifunctional metal-free electrode

materials. Electrochim. Acta 298, 210-218 (2019).

94 Hao, Q., Zhao, C. Q., Sun, B., Lu, C., Liu, J., Liu, M. J., Wan, L. J. & Wang,

D. Confined Synthesis of Two-Dimensional Covalent Organic Framework

Thin Films within Superspreading Water Layer. J. Am. Chem. Soc. 140, 12152-

12158 (2018).

95 Mukherjee, S. & Boudouris, B. W. in Organic Radical Polymers 1-15

(Springer, 2017).

96 Das, C. & Gebru, K. A. Polymeric Membrane Synthesis, Modification, and

Applications: Electro-Spun and Phase Inverted Membranes. (2018).

97 Pinnau, I. & Freeman, B. Formation and modification of polymeric

membranes: overview. Membrane Formation and Modification 744, 1-22

(2000).

98 Calabrò, V. & Basile, A. in Advanced Membrane Science and Technology for

Sustainable Energy and Environmental Applications 3-21 (Elsevier, 2011).

99 Li, G., Zhang, K. & Tsuru, T. Two-Dimensional Covalent Organic Framework

(COF) Membranes Fabricated via the Assembly of Exfoliated COF

Nanosheets. ACS Appl Mater Interfaces 9, 8433-8436 (2017).

100 Hao, D., Zhang, J., Lu, H., Leng, W., Ge, R., Dai, X. & Gao, Y. Fabrication of

a COF-5 membrane on a functionalized alpha-Al2O3 ceramic support using a

microwave irradiation method. Chem Commun (Camb) 50, 1462-1464 (2014).

101 Fan, H., Mundstock, A., Gu, J., Meng, H. & Caro, J. An azine-linked covalent

organic framework ACOF-1 membrane for highly selective CO2/CH4

separation. J. Mater. Chem. A 6, 16849-16853 (2018).

102 Shinde, D. B., Sheng, G., Li, X., Ostwal, M., Emwas, A. H., Huang, K. W. &

Lai, Z. Crystalline 2D Covalent Organic Framework Membranes for High-Flux

Organic Solvent Nanofiltration. J Am Chem Soc 140, 14342-14349 (2018).

103 Lu, H., Wang, C., Chen, J., Ge, R., Leng, W., Dong, B., Huang, J. & Gao, Y.

A novel 3D covalent organic framework membrane grown on a porous alpha-

Al2O3 substrate under solvothermal conditions. Chem Commun (Camb) 51,

15562-15565 (2015).

104 Wang, J., Si, L., Wei, Q., Hong, X., Cai, S. & Cai, Y. Covalent Organic

Frameworks as the Coating Layer of Ceramic Separator for High-Efficiency

Lithium–Sulfur Batteries. ACS Applied Nano Materials 1, 132-138 (2017).

105 Fu, J., Das, S., Xing, G., Ben, T., Valtchev, V. & Qiu, S. Fabrication of COF-

MOF Composite Membranes and Their Highly Selective Separation of

H2/CO2. J Am Chem Soc 138, 7673-7680 (2016).

106 Mullangi, D., Shalini, S., Nandi, S., Choksi, B. & Vaidhyanathan, R. Super-

hydrophobic covalent organic frameworks for chemical resistant coatings and

Bibliography 95

hydrophobic paper and textile composites. J. Mater. Chem. A 5, 8376-8384

(2017).

107 Sun, B., Zhu, C. H., Liu, Y., Wang, C., Wan, L. J. & Wang, D. Oriented

Covalent Organic Framework Film on Graphene for Robust Ambipolar

Vertical Organic Field-Effect Transistor. Chemistry of Materials 29, 4367-

4374 (2017).

108 Valentino, L., Matsumoto, M., Dichtel, W. R. & Marinas, B. J. Development

and Performance Characterisation of a Polyimine Covalent Organic

Framework Thin-Film Composite Nanofiltration Membrane. Environ Sci

Technol 51, 14352-14359 (2017).

109 Zou, C., Li, Q., Hua, Y., Zhou, B., Duan, J. & Jin, W. Mechanical Synthesis of

COF Nanosheet Cluster and Its Mixed Matrix Membrane for Efficient CO2

Removal. ACS Appl Mater Interfaces 9, 29093-29100 (2017).

110 Shan, M., Seoane, B., Rozhko, E., Dikhtiarenko, A., Clet, G., Kapteijn, F. &

Gascon, J. Azine-Linked Covalent Organic Framework (COF)-Based Mixed-

Matrix Membranes for CO2 /CH4 Separation. Chemistry 22, 14467-14470

(2016).

111 Kang, Z., Peng, Y., Qian, Y., Yuan, D., Addicoat, M. A., Heine, T., Hu, Z.,

Tee, L., Guo, Z. & Zhao, D. Mixed Matrix Membranes (MMMs) Comprising

Exfoliated 2D Covalent Organic Frameworks (COFs) for Efficient CO2

Separation. Chemistry of Materials 28, 1277-1285 (2016).

112 Yang, H., Wu, H., Yao, Z., Shi, B., Xu, Z., Cheng, X., Pan, F., Liu, G., Jiang,

Z. & Cao, X. Functionally graded membranes from nanoporous covalent

organic frameworks for highly selective water permeation. J. Mater. Chem. A

6, 583-591 (2018).

113 Shan, M., Seoane, B., Andres-Garcia, E., Kapteijn, F. & Gascon, J. Mixed-

matrix membranes containing an azine-linked covalent organic framework:

Influence of the polymeric matrix on post-combustion CO2-capture. J. Membr.

Sci. 549, 377-384 (2018).

114 Wu, X., Tian, Z., Wang, S., Peng, D., Yang, L., Wu, Y., Xin, Q., Wu, H. &

Jiang, Z. Mixed matrix membranes comprising polymers of intrinsic

microporosity and covalent organic framework for gas separation. J. Membr.

Sci. 528, 273-283 (2017).

115 Yang, H., Cheng, X., Cheng, X., Pan, F., Wu, H., Liu, G., Song, Y., Cao, X. &

Jiang, Z. Highly water-selective membranes based on hollow covalent organic

frameworks with fast transport pathways. J. Membr. Sci. 565, 331-341 (2018).

116 Yang, H., Wu, H., Pan, F., Li, Z., Ding, H., Liu, G., Jiang, Z., Zhang, P., Cao,

X. & Wang, B. Highly water-permeable and stable hybrid membrane with

asymmetric covalent organic framework distribution. J. Membr. Sci. 520, 583-

595 (2016).

117 Yang, H., Wu, H., Xu, Z., Mu, B., Lin, Z., Cheng, X., Liu, G., Pan, F., Cao, X.

& Jiang, Z. Hierarchical pore architectures from 2D covalent organic

nanosheets for efficient water/alcohol separation. J. Membr. Sci. 561, 79-88

(2018).

118 Wang, C., Li, Z., Chen, J., Li, Z., Yin, Y., Cao, L., Zhong, Y. & Wu, H.

Covalent organic framework modified polyamide nanofiltration membrane

with enhanced performance for desalination. J. Membr. Sci. 523, 273-281

(2017).

119 Peng, Y. W., Xu, G. D., Hu, Z. G., Cheng, Y. D., Chi, C. L., Yuan, D. Q.,

Cheng, H. S. & Zhao, D. Mechanoassisted Synthesis of Sulfonated Covalent

96 Bibliography

Organic Frameworks with High Intrinsic Proton Conductivity. ACS Appl.

Mater. Interfaces 8, 18505-18512 (2016).

120 Biswal, B. P., Kunjattu, S. H., Kaur, T., Banerjee, R. & Kharul, U. K.

Transforming covalent organic framework into thin-film composite

membranes for hydrocarbon recovery. Separation Science and Technology 53,

1752-1759 (2018).

121 Biswal, B. P., Chaudhari, H. D., Banerjee, R. & Kharul, U. K. Chemically

Stable Covalent Organic Framework (COF)-Polybenzimidazole Hybrid

Membranes: Enhanced Gas Separation through Pore Modulation. Chemistry

22, 4695-4699 (2016).

122 Shinde, D. B., Aiyappa, H. B., Bhadra, M., Biswal, B. P., Wadge, P.,

Kandambeth, S., Garai, B., Kundu, T., Kurungot, S. & Banerjee, R. A

mechanochemically synthesised covalent organic framework as a proton-

conducting solid electrolyte. J. Mater. Chem. A 4, 2682-2690 (2016).

123 Fan, H., Gu, J., Meng, H., Knebel, A. & Caro, J. High-Flux Membranes Based

on the Covalent Organic Framework COF-LZU1 for Selective Dye Separation

by Nanofiltration. Angew Chem Int Ed Engl 57, 4083-4087 (2018).

124 Pan, F., Wang, M., Ding, H., Song, Y., Li, W., Wu, H., Jiang, Z., Wang, B. &

Cao, X. Embedding Ag + @COFs within Pebax membrane to confer mass

transport channels and facilitated transport sites for elevated desulfurization

performance. J. Membr. Sci. 552, 1-12 (2018).

125 Chandra, S., Kandambeth, S., Biswal, B. P., Lukose, B., Kunjir, S. M.,

Chaudhary, M., Babarao, R., Heine, T. & Banerjee, R. Chemically stable

multilayered covalent organic nanosheets from covalent organic frameworks

via mechanical delamination. J Am Chem Soc 135, 17853-17861 (2013).

126 Cheng, Y., Ravi, S. K., Wang, Y., Tao, J., Gu, Y., Tan, S. C. & Zhao, D.

Covalent organic nanosheets with large lateral size and high aspect ratio

synthesised by Langmuir-Blodgett method. Chin. Chem. Lett. 29, 869-872

(2018).

127 Dey, K., Pal, M., Rout, K. C., Kunjattu, H. S., Das, A., Mukherjee, R., Kharul,

U. K. & Banerjee, R. Selective Molecular Separation by Interfacially

Crystallized Covalent Organic Framework Thin Films. J Am Chem Soc 139,

13083-13091 (2017).

128 Feldblyum, J. I., McCreery, C. H., Andrews, S. C., Kurosawa, T., Santos, E.

J., Duong, V., Fang, L., Ayzner, A. L. & Bao, Z. Few-layer, large-area, 2D

covalent organic framework semiconductor thin films. Chem Commun (Camb)

51, 13894-13897 (2015).

129 Khayum, M. A., Kandambeth, S., Mitra, S., Nair, S. B., Das, A., Nagane, S. S.,

Mukherjee, R. & Banerjee, R. Chemically Delaminated Free-Standing

Ultrathin Covalent Organic Nanosheets. Angew Chem Int Ed Engl 55, 15604-

15608 (2016).

130 Li, Y., Zhang, M., Guo, X., Wen, R., Li, X., Li, X., Li, S. & Ma, L. Growth of

high-quality covalent organic framework nanosheets at the interface of two

miscible organic solvents. Nanoscale Horizons 3, 205-212 (2018).

131 Ying, Y., Liu, D., Ma, J., Tong, M., Zhang, W., Huang, H., Yang, Q. & Zhong,

C. A GO-assisted method for the preparation of ultrathin covalent organic

framework membranes for gas separation. J. Mater. Chem. A 4, 13444-13449

(2016).

Bibliography 97

132 Zhang, N., Wang, T., Wu, X., Jiang, C., Chen, F., Bai, W. & Bai, R. Self-

exfoliation of 2D covalent organic frameworks: morphology transformation

induced by solvent polarity. RSC Adv. 8, 3803-3808 (2018).

133 Zhang, W., Zhang, L., Zhao, H., Li, B. & Ma, H. A two-dimensional cationic

covalent organic framework membrane for selective molecular sieving. J.

Mater. Chem. A 6, 13331-13339 (2018).

134 Warner, J. H., Schäffel, F., Bachmatiuk, A. & Rümmeli, M. H. in Graphene

(eds Jamie H. Warner, Franziska Schäffel, Alicja Bachmatiuk, & Mark H.

Rümmeli) 129-228 (Elsevier, 2013).

135 Cai, M., Thorpe, D., Adamson, D. H. & Schniepp, H. C. Methods of graphite

exfoliation. J. Mater. Chem. 22, 24992-25002 (2012).

136 Sasmal, H. S., Aiyappa, H. B., Bhange, S. N., Karak, S., Halder, A., Kurungot,

S. & Banerjee, R. Superprotonic Conductivity in Flexible Porous Covalent

Organic Framework Membranes. Angew Chem Int Ed Engl 57, 10894-10898

(2018).

137 Yao, B. J., Li, J. T., Huang, N., Kan, J. L., Qiao, L., Ding, L. G., Li, F. & Dong,

Y. B. Pd NP-Loaded and Covalently Cross-Linked COF Membrane

Microreactor for Aqueous CBs Dechlorination at Room Temperature. ACS

Appl Mater Interfaces 10, 20448-20457 (2018).

138 Properties of Solvents Table,

<https://www.sigmaaldrich.com/chemistry/stockroom-reagents/learning-

center/technical-library/solvent-properties.html> (2019).

139 The PubChem Project, <https://pubchem.ncbi.nlm.nih.gov/> (2018).

140 Properties of Solvents Used in Organic Chemistry,

<http://murov.info/orgsolvents.htm> (2018).

141 Phillips, J. M. Substrate selection for thin-film growth. MRS Bull. 20, 35-39

(1995).

142 Moore, A. Highly oriented pyrolytic graphite and its intercalation compounds.

Chemischer Informationsdienst 12, no-no (1981).

143 (ed Highly ordered pyrolytic graphite (HOPG)) (SPI Supplies, 2019).

144 (ed HOPG) (AIST-NT, 2019).

145 (ed Monolayer of Graphene on Cu) (Graphenea, 2019).

146 (ed Microscope Glass Slide) (Wiltronics, 2019).

147 (ed Silicon Chip Specimen Supports) (Ted Pella, 2019).

148 (ed Porcelain High Form Crucibles) (Thomas Scientific, 2019).

149 Sinturel, C., Vayer, M. n., Morris, M. & Hillmyer, M. A. Solvent vapour

annealing of block polymer thin films. Macromolecules 46, 5399-5415 (2013).

150 Miao, J., Chen, H., Liu, F., Zhao, B., Hu, L., He, Z. & Wu, H. Efficiency

enhancement in solution-processed organic small molecule: Fullerene solar

cells via solvent vapour annealing. Appl. Phys. Lett. 106, 49_41 (2015).

151 Jung, B., Kim, K. & Kim, W. Microwave-assisted solvent vapour annealing to

rapidly achieve enhanced performance of organic photovoltaics. J. Mater.

Chem. A 2, 15175-15180 (2014).

152 Mason, T. J. & Tiehm, A. Advances in Sonochemistry: Ultrasound in

Environmental Protection. Vol. 6 (Elsevier, 2001).

153 Freitas, S., Hielscher, G., Merkle, H. P. & Gander, B. Continuous contact-and

contamination-free ultrasonic emulsification—a useful tool for pharmaceutical

development and production. Ultrason. Sonochem. 13, 76-85 (2006).

98 Bibliography

154 Lee, B., Park, Y.-H., Hwang, Y.-T., Oh, W., Yoon, J. & Ree, M. Ultralow-k

nanoporous organosilicate dielectric films imprinted with dendritic spheres.

Nat. Mater. 4, 147 (2005).

155 Warren, W., Dimos, D., Pike, G., Tuttle, B., Raymond, M., Ramesh, R. &

Evans Jr, J. Voltage shifts and imprint in ferroelectric capacitors. Appl. Phys.

Lett. 67, 866-868 (1995).

156 Wang, Y. & Liang, Z. Solvent effects and its role in quantitatively

manipulating the crystal growth: benzoic acid as case study. Crystengcomm 19,

3198-3205 (2017).

157 Stoica, C., Verwer, P., Meekes, H., Van Hoof, P., Kaspersen, F. & Vlieg, E.

Understanding the effect of a solvent on the crystal habit. Cryst. Growth Des.

4, 765-768 (2004).

158 Yang, S.-T., Kim, J., Cho, H.-Y., Kim, S. & Ahn, W.-S. Facile synthesis of

covalent organic frameworks COF-1 and COF-5 by sonochemical method.

RSC Adv. 2, 10179-10181 (2012).

159 Peshkovsky, A. S., Peshkovsky, S. L. & Bystryak, S. Scalable high-power

ultrasonic technology for the production of translucent nanoemulsions.

Chemical Engineering and Processing: Process Intensification 69, 77-82

(2013).

160 Schubert, D. W. & Dunkel, T. Spin coating from a molecular point of view: its

concentration regimes, influence of molar mass and distribution. Materials

Research Innovations 7, 314-321 (2003).

161 Chernikova, V., Shekhah, O. & Eddaoudi, M. Advanced fabrication method

for the preparation of MOF thin films: liquid-phase epitaxy approach meets

spin coating method. ACS Appl. Mater. Interfaces 8, 20459-20464 (2016).

162 Bornside, D., Macosko, C. & Scriven, L. Spin coating: One‐dimensional

model. J. Appl. Phys. 66, 5185-5193 (1989).

163 Xia, Z. C. & Hutchinson, J. W. Crack patterns in thin films. J. Mech. Phys.

Solids 48, 1107-1131 (2000).

164 Ladewig, B. & Al-Shaeli, M. N. Z. Fundamentals of Membrane Processes. 13-

37 (2017).

165 Nunes, S. P. & Peinemann, K.-V. Membrane technology: in the chemical

industry. (John Wiley & Sons, 2006).

166 Zhou, W., Wu, H. & Yildirim, T. Structural stability and elastic properties of

prototypical covalent organic frameworks. Chem. Phys. Lett. 499, 103-107

(2010).

167 Cammarata, R. & Sieradzki, K. Effects of surface stress on the elastic moduli

of thin films and superlattices. Phys. Rev. Lett. 62, 2005 (1989).

168 Huang, D. Size-dependent response of ultra-thin films with surface effects.

International Journal of Solids and Structures 45, 568-579 (2008).

169 Liang, X., Wang, B. & Liu, Y. Thickness effect of a thin film on the stress field

due to the eigenstrain of an ellipsoidal inclusion. International Journal of

Solids and Structures 46, 322-330 (2009).

170 Domke, J. & Radmacher, M. Measuring the elastic properties of thin polymer

films with the atomic force microscope. Langmuir 14, 3320-3325 (1998).

171 Den Toonder, J., Van Dommelen, J. & Baaijens, F. The relation between single

crystal elasticity and the effective elastic behaviour of polycrystalline

materials: theory, measurement and computation. Modelling and Simulation in

Materials Science and Engineering 7, 909 (1999).

Bibliography 99

172 Albert, J. N., Young, W.-S., Lewis III, R. L., Bogart, T. D., Smith, J. R. & Epps

III, T. H. Systematic study on the effect of solvent removal rate on the

morphology of solvent vapour annealed ABA triblock copolymer thin films.

ACS Nano 6, 459-466 (2011).

100 Appendices

Appendices

Appendix A

Effects of Ultrasonic Vibration on Film Morphology

Figure 63 presents the setup for ultrasonic vibration assisted drop-casting of

solution on a substrate. The blank substrate is first fixed on the carbon tape. The

ultrasonic bath is then turned on and the solution drop-casted on the substrate. The

droplet will 'fizzle' systematically and as soon as the droplet disappears, the bath is

switched off and the sample moved to the next appropriate apparatus. The difference

between a sample with the solution drop-cast using this method and without post-

synthesis can be seen in Figure 64.

Figure 63. Setup of ultrasonic vibration assisted drop-casting.

Appendices 101

Figure 64. Comparison of a COF film synthesised via room temperature solvent-vapour assisted

annealing with solution drop-casted (a) and (b) with assistance from ultrasonic vibrations and (c) and

(d) without.

300 µm

300 µm

2 mm

2 mm

(a)

(d)

(c)

(b)

Fig

ure

80.

Co

mpa

riso

n of

a

CO

F

film

synt

hesi

sed

via

roo

m

tem

pera

ture

solv

ent-

vap

our

assi

sted

ann

eali

ng

with

solu

tion

dro

p-

cast

ed

(a)

and

(b)

with

assi

stan

ce

fro

m

ultr

aso

nic

vibr

atio

ns

and

(c)

and

(d)

with

out.

(b)

102 Appendices

Appendix B

COF-1: Replication of Côté et al.’s 1 method

Although our objective was to synthesise COF-1 in film form, we also made an

attempt to replicate the synthesis technique as described by Côté et al.1 for COF-1 bulk

powder. The setup is shown in Figure 65. We tested two different sized Schlenk tubes,

with both being unsuccessful as shown in the IR spectra in Figure 66, which shows

that the traces overlap perfectly with that of the precursor powder. We surmised the

failure to the following reasons:

1. Moisture may have entered at one point of the apparatus.

2. Slight overtime may have caused the reversal of COF-1 formation, but

this then must mean that this event happened very quickly. More

experiments were needed to confirm this theory.

3. Pre-existing moisture in solvents and/or precursor molecule.

(a)

(b)

(c)

Figure 65. Setup of COF-1 bulk powder synthesis via solvothermal processes.

Appendices 103

Figure 66. IR spectra of bulk powders synthesised via Côte, et al.’s1 procedure.

104 Appendices

Appendix C

For future studies: Plasma treatment of solvothermal films

The objective of this experiment was to utilise the process of carbonisation to

reduce the COF-1 band gap and eliminate the boroxine ring that makes COF-1

vulnerable in humid conditions by plasma treating the COF at room temperature and

atmospheric pressure. The sample is a COF-1 film synthesised partially via

solvothermal annealing to assure maximum film adhesion using the

cyclohexanone/ethanol/ether solvent mixture. We were able to induce morphological

changes on the film as shown but not as uniformly as we expected (Figure 67). This

could be due to the pre-existing inhomogeneity of the COF-1 film (e.g., due to

incomplete conversion and/or solvent retention) and will require further investigations.

(a)

(d)

(b)

(c)

Figure 67. Film and crystal morphology of COF-1 film synthesised via partial solvothermal annealing

and then plasma treatment. (a) film morphology after plasma-treatment; (b) colour variations of

crystals; (c) white specks appearing to outline crystal shapes; (d) dendritic-like darkening of

crystals.

3 mm

400 µm

400 µm

400 µm

Appendices 105

Appendix D

Other Morphologies Observed from Vapour Annealed COF-1 on Si Wafer

Using the Cyclohexanone/Ethanol/Ether Solution

Other Morphologies Observed from Vapour-Annealed COF-1 on Si Wafer

Using the Cyclohexanone/Ethanol/Ether Solution

(d)

(b)

(c)

(e)

(f)

(a)

Figure 68. Other crystal morphologies observed on a COF-1 film synthesised via solvent vapour annealing on a

Si wafer. (a) crystal morphology of overall film; (b) crystal morphology of the more homogenous regions; (c)

close-up of pointed edges of (b); (d) radiating agglomerates of spindle-like crystals; (e) layered, intergrowth

of shell-like structures; (f) circular-disk shaped agglomerates of crystals.

15 µm

10 µm

3 µm

30 µm

15 µm

150 µm

106 Appendices

Appendix E

Synopsis of Parameters Explored in Thesis

In detail

1. Synthesis method

a. Drop-casting

b. Solvothermal annealing

c. Sonochemical annealing

d. Solvent-vapour annealing

e. Spin-coating

f. Thermal imprinting

2. Solvents

a. Acetone

b. Cyclohexanone

c. Cyclopentanone

d. Diethyl ether

e. Ethanol

f. Heptanoic acid

3. Precursor concentration

a. 1 mg to 55 mg per 1.5 ml

4. Substrates

a. Ceramic crucible

b. Ceramic filter paper

c. Teflon filter paper

d. Glass slides

e. Graphene on Cu

f. HOPG

g. SiO₂ on Si wafer

Briefly explored (further discussed in Chapter 4 conclusion)

5. Temperature

6. Time

7. Water vapour

8. Apparatus

9. Pressure

10. Humidity

11. Deposition consistency

synthesis and characterisation of covalent organic ... · organic frameworks that were...

Documents